"""Sedona SAX Tree View Viewer & Program Editor — UI wiring, dialogs and event handlers."""

import os
import sys
import tkinter as tk
from tkinter import filedialog, font as tkfont, messagebox, ttk

import threading

import app_state
import connect_dialog
import device_session
import link_view
import manage_kits
import palette
import properties
import sax_file
import schema
import undo
import wiresheet
from sax_dom import (find_component_by_path, find_parent_element, get_component_path,
                     read_meta_property, update_meta_property)

# Tree column measurements, in pixels. The padding covers the expander arrow and the
# cell's own inset, which sit ahead of the text ttk lets us measure.
TREE_INDENT = 20
TREE_TEXT_PADDING = 28
TREE_MIN_COLUMN_WIDTH = 40

# Rows each list asks for. This is the only real floor ttk offers on the left column's own
# split: a pane keeps its requested height, and a short window takes the shortfall from the
# panes in proportion to their weights. Left at the ten-row default the palette asked for
# so much that the tree, carrying the larger weight, gave up all of its height first.
TREE_REQUESTED_ROWS = 6


class _ToolTip:
    """Minimal hover tooltip for toolbar widgets."""

    def __init__(self, widget, text):
        self._widget = widget
        self._text = text
        self._tw = None
        widget.bind("<Enter>", self._show, add="+")
        widget.bind("<Leave>", self._hide, add="+")

    def _show(self, _event=None):
        x = self._widget.winfo_rootx() + self._widget.winfo_width() // 2
        y = self._widget.winfo_rooty() + self._widget.winfo_height() + 4
        tw = tk.Toplevel(self._widget)
        tw.wm_overrideredirect(True)
        tw.wm_geometry(f"+{x}+{y}")
        label = tk.Label(
            tw, text=self._text, background="#ffffe0", foreground="#000000",
            relief="solid", borderwidth=1, padx=4, pady=2,
            font=("Segoe UI", 8),
        )
        label.pack()
        self._tw = tw

    def _hide(self, _event=None):
        if self._tw:
            self._tw.destroy()
            self._tw = None
PALETTE_REQUESTED_ROWS = 5

# Below this the shortfall is deep enough to shrink the tree past a usable few rows, and
# ttk has no way to hold a pane at a floor once the room is simply not there.
MIN_WINDOW_HEIGHT = 300

# How many visited views Back and Forward can walk through. A session touches few of
# them; the cap only stops an unbounded list.
HISTORY_LIMIT = 50

# What a history entry can be showing. The editor pane holds one of these three at a
# time, so the history that walks it has to be able to name each of them.
VIEW_SHEET = "sheet"
VIEW_PROPERTIES = "properties"
VIEW_LINKS = "links"

# Tk class names of the widgets that own the keyboard while they have focus. Del and
# Backspace mean "edit this text" in each of them, never "delete the selected component".
TEXT_ENTRY_CLASSES = ("Entry", "TEntry", "TCombobox", "Text", "Spinbox", "TSpinbox")

MENU_BACK_LABEL = "Back"
MENU_FORWARD_LABEL = "Forward"

# --- Editor pane history ---
# Each entry is (kind, element), naming what the pane was showing:
#
#   (VIEW_SHEET, element)       the sheet whose children are painted, which is what
#                               app_state.current_sheet_parent holds; None is the
#                               file's root node
#   (VIEW_PROPERTIES, comp)     the Properties view of that component
#   (VIEW_LINKS, comp)          the Link View of that component
#
# Opening a Properties or Link view is a move like opening a sheet, so Back from one
# lands on what the pane showed before it rather than skipping past to the sheet before
# that, and Forward brings it back.
#
# Elements are kept rather than tree item ids because a rebuild of the tree hands out
# fresh ids, while the elements themselves survive anything short of a delete or a new
# file.
view_history = []
history_index = -1          # position in view_history that is on screen
_suppress_history = False   # set while Back or Forward restores an entry
_history_file_path = None   # the file view_history was recorded against

# Filled in once the widgets exist. History can be recorded before then, so every
# function that touches them checks first.
btn_back = None
btn_forward = None
btn_toggle_pane = None
zoom_label = None
menu_view = None
show_left_column_var = None

# The Properties, Link and Kits views each take the editor pane over from the canvas
# rather than floating above it, so at most one of the four is on screen and these three
# say which. All None means the sheet is showing.
properties_panel = None
link_panel = None
kits_panel = None

# The sheet to put the tree back on when a sheet change is refused because of unsaved
# edits, and the flag that stops the restoring selection from being read as a fresh
# choice. The element is held rather than the tree item id, for the same reason the
# history holds elements: a rebuild of the tree hands out fresh ids.
panel_return_sheet = None
_restoring_selection = False

# The link a jump has asked to be picked out, held until the sheet it is on has been
# painted. Selecting a tree node paints its sheet, and painting a sheet clears whatever
# link was selected, so the pick cannot be made at the point the node is selected --
# see go_to_sheet_with_link.
_pending_link_selection = None


def anything_unwritten():
    """Whether there is a change no one has written yet, in the document or in a field.

    The Properties view counts before its Save: a typed value is as unwritten as a
    dragged box, and a file name that only went red on the second of those would be
    telling half the story.
    """
    if app_state.is_dirty:
        return True
    return properties_panel is not None and properties_panel.dirty


def refresh_dirty_display():
    """Puts the file name on the tree's root row in red, or back to its usual colour.

    The tag is added to or taken off the row rather than having its colour reconfigured.
    Reconfiguring a tag does not reliably repaint rows already on screen, so the name
    only turned red on the paths that happened to rebuild the tree -- a Properties Save
    did, dragging a box did not.
    """
    tree = app_state.tree
    if tree is None:
        return

    rows = tree.get_children()
    if not rows:
        return

    root_row = rows[0]
    wanted = (app_state.DIRTY_NAME_TAG,) if anything_unwritten() else ()
    if tuple(tree.item(root_row, "tags")) != wanted:
        tree.item(root_row, tags=wanted)


def refresh_all_views():
    """Rebuilds tree and canvas from the DOM. Registered as undo/redo's view refresh."""
    sax_file.refresh_treeview_from_dom(app_state.xml_root_element)
    wiresheet.render_current_sheet()

    # An undo or a cascading delete can take the component a panel is showing out of the
    # document underneath it. Nothing it could display then would be true.
    if properties_panel is not None and not _component_in_document(properties_panel.comp):
        close_editor_panel(force=True)

    # The Link View reads the document and writes nothing, so a change elsewhere is
    # simply redrawn rather than being a reason to take the view down.
    if link_panel is not None:
        if _component_in_document(link_panel.comp):
            link_panel.draw()
        else:
            close_editor_panel(force=True)


def _component_in_document(comp):
    root = app_state.xml_root_element
    return root is not None and any(element is comp for element in root.iter())


def delete_component(comp):
    """Deletes a component, taking a view of it down with it if one is showing.

    Every delete in the UI comes through here. sax_file does its own view refresh rather
    than going through refresh_all_views, so the panel would otherwise be left describing
    a component the document no longer has.
    """
    panel = properties_panel or link_panel
    showing = panel is not None and panel.comp is comp
    if sax_file.execute_cascading_deletion(comp) and showing:
        close_editor_panel(force=True)


def editor_panel_showing():
    """Whether something other than the wiresheet has the editor pane."""
    return properties_panel is not None or link_panel is not None or kits_panel is not None


def _take_editor_pane():
    """Hides the sheet so a panel can have the pane.

    The canvas is only unpacked, never destroyed, so the sheet keeps its items and its
    scroll position and comes back exactly as it was. It is still painted while it is
    hidden -- a Properties Save repaints it -- which costs nothing and means there is no
    catching up to do when it returns.
    """
    global panel_return_sheet
    panel_return_sheet = app_state.current_sheet_parent

    canvas_vscroll.pack_forget()
    canvas_hscroll.pack_forget()
    app_state.canvas.pack_forget()


def _give_editor_pane_back():
    """Puts the sheet back, in the order the pane was built in.

    The scrollbars are repacked before the canvas because pack honours the order it was
    given: packed after it, they would take their space from the middle of the pane.
    """
    canvas_vscroll.pack(side=tk.RIGHT, fill=tk.Y)
    canvas_hscroll.pack(side=tk.BOTTOM, fill=tk.X)
    app_state.canvas.pack(fill=tk.BOTH, expand=True)
    app_state.clear_status()


def show_properties(comp):
    """Puts one component's slots in the editor pane, in place of the wiresheet."""
    global properties_panel
    if comp is None:
        return

    # Right-clicking a second component is a way of leaving the first, so it asks about
    # unsaved edits exactly as Close does, and a refusal leaves the first one showing.
    if not close_editor_panel():
        return

    _take_editor_pane()
    properties_panel = properties.build_properties_panel(
        frame_editor, comp, on_change=refresh_all_views, on_close=dismiss_properties,
        on_dirty_change=refresh_dirty_display,
    )
    properties_panel.frame.pack(fill=tk.BOTH, expand=True)
    record_visit(VIEW_PROPERTIES, comp)
    app_state.set_status(f"Properties: {comp.get('name', '?')}")


def dismiss_properties(restore_canvas=True):
    """Takes the Properties view down and gives the pane back, asking nothing.

    The panel's own Close is what asks; by the time it calls this the question has been
    put and answered. Anything that has made the view moot -- the component deleted, the
    file closed -- comes straight here too.
    """
    global properties_panel
    if properties_panel is None:
        return

    properties_panel.destroy()
    properties_panel = None
    refresh_dirty_display()

    if restore_canvas:
        _give_editor_pane_back()


def show_link_view(comp):
    """Puts one component's links in the editor pane, in place of the wiresheet.

    The same trade the Properties view makes, and for the same reason: the sheet and a
    picture of one component's wiring are two answers to the same question, so only one
    of them should be on screen at a time.
    """
    global link_panel
    if comp is None:
        return

    if not close_editor_panel():
        return

    _take_editor_pane()
    link_panel = link_view.build_link_view_panel(
        frame_editor, comp, on_open_link=open_link_from_link_view,
        on_close=dismiss_link_view,
    )
    link_panel.frame.pack(fill=tk.BOTH, expand=True)
    record_visit(VIEW_LINKS, comp)
    app_state.set_status(f"Link View: {comp.get('name', '?')}")


def dismiss_link_view(restore_canvas=True):
    """Takes the Link View down and gives the pane back.

    Nothing is ever asked here: the view has changed nothing, so there is nothing to
    lose by leaving it.
    """
    global link_panel
    if link_panel is None:
        return

    link_panel.destroy()
    link_panel = None

    if restore_canvas:
        _give_editor_pane_back()


def show_kits():
    """Puts the kit table in the editor pane, in place of the wiresheet."""
    global kits_panel
    if app_state.xml_root_element is None:
        messagebox.showinfo("Manage Kits", "Open a SAX file first.")
        return
    if app_state.xml_root_element.find("schema") is None:
        messagebox.showwarning("Manage Kits", "This file has no <schema> block to manage.")
        return

    if not close_editor_panel():
        return

    _take_editor_pane()
    kits_panel = manage_kits.build_kits_panel(
        frame_editor, on_applied=refresh_palette_kits, on_close=dismiss_kits,
    )
    kits_panel.frame.pack(fill=tk.BOTH, expand=True)
    app_state.set_status("Manage Kits")


def dismiss_kits(restore_canvas=True):
    """Takes the Kits view down and gives the pane back."""
    global kits_panel
    if kits_panel is None:
        return

    kits_panel.destroy()
    kits_panel = None

    if restore_canvas:
        _give_editor_pane_back()


def close_editor_panel(force=False):
    """Frees the editor pane, asking about unsaved edits first. True if it is now free.

    Every way of leaving a panel goes through this, so "you have not saved that" is
    asked once, in one place, whether the user pressed Close, picked another sheet,
    walked the history or opened a different component. Only the Properties view has
    anything to ask about; other views simply stand down.
    """
    if link_panel is not None:
        dismiss_link_view()
        return True
    if kits_panel is not None:
        dismiss_kits()
        return True
    if properties_panel is None:
        return True
    if force:
        dismiss_properties()
        return True
    return bool(properties_panel.close())


def _restore_tree_selection():
    """Puts the tree back on the sheet that is actually showing, after a refused change."""
    global _restoring_selection
    item_id = _tree_item_for_element(panel_return_sheet)
    if item_id is None:
        return

    _restoring_selection = True
    app_state.tree.selection_set(item_id)
    # ttk queues its selection event rather than dispatching it, so the flag has to
    # outlive this call or the restoring selection is read as a fresh choice.
    app_state.window.after_idle(_clear_restoring_flag)


def _clear_restoring_flag():
    global _restoring_selection
    _restoring_selection = False


def do_undo(event=None):
    if app_state.live_mode:
        app_state.set_status("Undo is not available in live mode", warn=True)
        return "break"
    label = undo.undo()
    if label is None:
        return "break"
    app_state.window.title(f"{app_state.base_window_title()} — undid {label}")
    return "break"


def do_redo(event=None):
    if app_state.live_mode:
        app_state.set_status("Redo is not available in live mode", warn=True)
        return "break"
    label = undo.redo()
    if label is None:
        return "break"
    app_state.window.title(f"{app_state.base_window_title()} — redid {label}")
    return "break"


def do_save(event=None):
    if app_state.live_mode:
        do_download_to_sax()
        return "break"
    sax_file.execute_direct_overwrite_save()
    return "break"


def do_download_to_sax():
    """In live mode, saves the current device DOM snapshot as a .sax file."""
    if app_state.xml_root_element is None:
        return
    path = filedialog.asksaveasfilename(
        title="Download to SAX",
        defaultextension=".sax",
        filetypes=[("Sedona SAX", "*.sax"), ("All files", "*.*")],
        initialfile=f"{app_state.device_host}.sax"
    )
    if not path:
        return
    try:
        sax_file.write_sax_to_path(path)
        app_state.set_status(f"Downloaded to {os.path.basename(path)}")
    except Exception as e:
        messagebox.showerror("Download Failed", str(e))


NL = chr(10)   # newline, for the multi-line message boxes


def do_tidy_layout(event=None):
    """Menu command: re-flow the open sheet so nothing overlaps."""
    if not app_state.canvas_comp_map:
        messagebox.showinfo("Tidy Layout", "Open a sheet with components first.")
        return "break"

    moved, clamped = wiresheet.tidy_layout()
    multiplier = app_state.meta_scale // app_state.GRID_SIZE

    if moved == 0:
        messagebox.showinfo("Tidy Layout", "Nothing to move — no boxes overlap.")
    elif clamped:
        messagebox.showwarning(
            "Tidy Layout",
            f"Repositioned {moved} component(s).\n\n"
            f"{clamped} still hit the coordinate limit of {wiresheet.META_COORD_MAX} "
            "steps, because meta stores x and y in one byte each. This sheet is already "
            f"laid out at {multiplier} grid squares per step, the widest meta can reach, "
            "so it does not fit however it is spaced. Split it across sub-sheets."
        )
    elif multiplier > 1:
        messagebox.showinfo(
            "Tidy Layout",
            f"Repositioned {moved} component(s)." + NL + NL +
            f"This sheet needs more width than meta's {wiresheet.META_COORD_MAX} "
            f"coordinate steps allow at one grid square each, so it is laid out at "
            f"{multiplier} grid squares per step ({app_state.meta_scale} px). Boxes "
            f"settle on every {multiplier} grid lines rather than every one. Your grid "
            "size in Preferences is unchanged."
        )
    return "break"


# --- Zoom ---

def update_zoom_label():
    """Puts the painted scale on the toolbar as a percentage."""
    if zoom_label is not None:
        zoom_label.config(text=f"{round(app_state.zoom * 100)}%")


def zoom_to(level, anchor=None):
    """Repaints the sheet at this zoom, holding one point of it still on screen.

    ``anchor`` is a canvas-relative (x, y) to keep under the same pixel -- the pointer,
    for a wheel zoom -- and defaults to the middle of the visible area, so a menu or
    keyboard zoom closes in on what is already being looked at rather than on the
    top-left corner.
    """
    canvas = app_state.canvas
    before = app_state.zoom

    if anchor is None:
        anchor = (canvas.winfo_width() / 2, canvas.winfo_height() / 2)

    # Where the held point sits on the unscaled sheet.
    sheet_x = canvas.canvasx(anchor[0]) / before
    sheet_y = canvas.canvasy(anchor[1]) / before

    after = wiresheet.set_zoom(level)
    update_zoom_label()
    if after == before:
        return

    # Scroll the repainted sheet back under the held point. The scrollregion starts at
    # the origin, so its far corner is the span the view fractions are measured against.
    region = str(canvas.cget("scrollregion")).split()
    if len(region) == 4:
        span_x = float(region[2]) or 1.0
        span_y = float(region[3]) or 1.0
        canvas.xview_moveto(max(0.0, (sheet_x * after - anchor[0]) / span_x))
        canvas.yview_moveto(max(0.0, (sheet_y * after - anchor[1]) / span_y))


def do_zoom_in(event=None):
    zoom_to(wiresheet.zoom_step(1))
    return "break"


def do_zoom_out(event=None):
    zoom_to(wiresheet.zoom_step(-1))
    return "break"


def do_zoom_reset(event=None):
    zoom_to(1.0)
    return "break"


def do_zoom_fit(event=None):
    """Zoom so the whole sheet is visible, and scroll it into view.

    The fitted scale is not one of the ladder's, so the percentage on the toolbar is
    where it is read off; stepping in or out from there rejoins the ladder.
    """
    canvas = app_state.canvas
    level = wiresheet.fit_zoom_level()
    bounds = wiresheet.sheet_content_bounds()

    if level is None or bounds is None:
        app_state.set_status("Nothing to fit — open a sheet with components first.")
        return "break"

    applied = wiresheet.set_zoom(level)
    update_zoom_label()

    # Bring the sheet's own top-left corner to the top-left of the view, since a fitted
    # sheet is measured from there rather than from the canvas origin.
    region = str(canvas.cget("scrollregion")).split()
    if len(region) == 4:
        span_x = float(region[2]) or 1.0
        span_y = float(region[3]) or 1.0
        canvas.xview_moveto(max(0.0, (bounds[0] * applied - wiresheet.FIT_PADDING) / span_x))
        canvas.yview_moveto(max(0.0, (bounds[1] * applied - wiresheet.FIT_PADDING) / span_y))
    return "break"


def on_canvas_wheel_zoom(event):
    """Ctrl + wheel over the sheet, the gesture every other editor uses for zoom."""
    if not event.delta:
        return "break"
    step = 1 if event.delta > 0 else -1
    zoom_to(wiresheet.zoom_step(step), anchor=(event.x, event.y))
    return "break"


# --- Event handlers ---

def on_tree_select(event):
    """Triggered when a branch node is clicked in the Navigation view."""
    global _pending_link_selection, _restoring_selection

    # Read and cleared first thing, so a link a jump asked for is only ever picked out
    # by the very selection that jump made. A selection that turns out not to paint a
    # sheet -- one being put back after a refusal, one of nothing at all -- drops the
    # request rather than leaving it to be applied to some later sheet.
    pending_link = _pending_link_selection
    _pending_link_selection = None

    if _restoring_selection:
        return

    # Choosing a node means "show me that sheet", so whatever has the pane stands down
    # rather than leaving the selection looking like it did nothing. Refusing to leave
    # has to put the tree back on the sheet that is still showing.
    if not close_editor_panel():
        _restore_tree_selection()
        return

    tree = app_state.tree
    selected_items = tree.selection()
    if not selected_items:
        return

    selected_id = selected_items[0]
    element = app_state.tree_element_map.get(selected_id)
    components_to_render = []

    # Remembered so undo/redo and tidy can repaint this same sheet from the DOM.
    app_state.current_sheet_parent = element
    record_visit(VIEW_SHEET, element)

    if app_state.live_mode and app_state.device_session and element is not None:
        comp_id_str = element.get("id")
        if comp_id_str is not None:
            ds = app_state.device_session
            ds.close_wiresheet()
            try:
                ds.open_wiresheet(int(comp_id_str))
                sax_root = ds.build_dom_tree(ds.sox_client.cache_get(0))
                ds.build_links_elements(sax_root)
                app_state.xml_root_element = sax_root
                sax_file.refresh_treeview_from_dom(sax_root)
                # re-find element by comp id after DOM rebuild (tree item IDs changed)
                element = None
                for tid, elem in app_state.tree_element_map.items():
                    if elem.get("id") == comp_id_str:
                        element = elem
                        selected_id = tid
                        break
                if element is not None:
                    _restoring_selection = True
                    tree.selection_set(selected_id)
                    tree.see(selected_id)
                    tree.after_idle(_clear_restoring_flag)
                app_state.current_sheet_parent = element
            except Exception:
                import traceback; traceback.print_exc()

        # The DOM rebuild replaced every element object, so a pending link from the
        # old tree can no longer be found by identity.  Re-resolve it by from/to.
        if pending_link is not None:
            old_from = pending_link.get("from", "")
            old_to = pending_link.get("to", "")
            links_container = app_state.xml_root_element.find("links") if app_state.xml_root_element is not None else None
            if links_container is not None:
                for lnk in links_container:
                    if lnk.get("from") == old_from and lnk.get("to") == old_to:
                        pending_link = lnk
                        break

    if element is not None:
        components_to_render = [child for child in element if child.tag == "comp"]
    elif tree.exists(selected_id):
        top_level_ids = tree.get_children(selected_id)
        for t_id in top_level_ids:
            comp_elem = app_state.tree_element_map.get(t_id)
            if comp_elem is not None:
                components_to_render.append(comp_elem)

    wiresheet.render_components(components_to_render)

    # The sheet is painted now, so a link the jump asked for has wires to be found among.
    if pending_link is not None:
        select_link_on_open_sheet(pending_link)


def delete_selected_link():
    """Removes the currently selected link from the DOM and repaints."""
    if not app_state.selected_link_tag:
        return
    link_elem = app_state.canvas_link_map.get(app_state.selected_link_tag)
    if link_elem is None:
        return

    from_ref = link_elem.get("from", "")
    to_ref = link_elem.get("to", "")
    confirm = messagebox.askyesno(
        "Delete Link",
        f"Delete link?\n\n  from: {from_ref}\n  to:     {to_ref}"
    )
    if not confirm:
        return

    sax_file.remove_link(link_elem)


def link_endpoint_components(link_elem):
    """The two components a link joins, resolved from its from/to references."""
    root = app_state.xml_root_element
    ends = []
    for attribute in ("from", "to"):
        reference = link_elem.get(attribute, "")
        path = reference.rsplit(".", 1)[0] if "." in reference else ""
        ends.append(find_component_by_path(root, path))
    return ends


def link_target_sheet(link_elem):
    """Where a link leads: (sheet element, the component at that end), or None.

    The far end is the one that is not on the open sheet, which is exactly the end a
    knob stands in for. A link with both ends on this sheet has nowhere to go, and
    neither has one whose references name a component this document does not have.
    """
    root = app_state.xml_root_element
    if root is None or link_elem is None:
        return None

    here = sax_file.current_sheet_container()
    for component in link_endpoint_components(link_elem):
        if component is None:
            continue
        sheet = find_parent_element(root, component)
        if sheet is not None and sheet is not here:
            return sheet, component
    return None


def tree_item_for_sheet(sheet_element):
    """The navigation tree item that opens this sheet, or None if it has no node.

    The app element is the file's root node, which the tree holds without an entry in
    the element map, and which the history records as None for the same reason.
    """
    root = app_state.xml_root_element
    app_element = root.find("app") if root is not None else None
    if sheet_element is app_element or sheet_element is root:
        return _tree_item_for_element(None)
    return _tree_item_for_element(sheet_element)


def scroll_canvas_to(tag):
    """Scrolls the sheet so the tagged items sit in the middle of the view."""
    canvas = app_state.canvas
    box = canvas.bbox(tag)
    region = str(canvas.cget("scrollregion")).split()
    if box is None or len(region) != 4:
        return

    span_x = float(region[2]) or 1.0
    span_y = float(region[3]) or 1.0
    middle_x = (box[0] + box[2]) / 2 - canvas.winfo_width() / 2
    middle_y = (box[1] + box[3]) / 2 - canvas.winfo_height() / 2
    canvas.xview_moveto(max(0.0, middle_x / span_x))
    canvas.yview_moveto(max(0.0, middle_y / span_y))


def select_link_element(link_elem):
    """Selects whichever wire or knob now stands for this link, and scrolls it into view.

    A repaint hands out fresh wire tags, so the link is found again by its element
    rather than by the tag it had on the sheet it was picked on.
    """
    for wire_tag, painted in app_state.canvas_link_map.items():
        if painted is link_elem:
            wiresheet.select_link(wire_tag)
            scroll_canvas_to(wire_tag)
            return True
    return False


def go_to_link_sheet():
    """Link popup: open the sheet the far end of this link is on, with the link selected."""
    link_elem = app_state.canvas_link_map.get(app_state.selected_link_tag)
    destination = link_target_sheet(link_elem) if link_elem is not None else None
    if destination is None:
        app_state.set_status("This link has no other sheet to go to.", warn=True)
        return

    go_to_sheet_with_link(destination[0], link_elem)


def go_to_sheet_with_link(sheet_element, link_elem):
    """Opens a sheet and picks out one link on it.

    Shared by the link popup's jump and the Link View, so following a link lands the
    same way whichever of them was used to follow it.
    """
    global _pending_link_selection

    item_id = tree_item_for_sheet(sheet_element)
    if item_id is None:
        app_state.set_status(
            f"'{link_view.sheet_display_name(sheet_element)}' is not in the "
            "navigation tree.",
            warn=True,
        )
        return

    # Selecting the node is what paints the sheet, and painting a sheet clears whatever
    # link was selected -- so the link is left as a request for the selection handler to
    # honour once it has painted, rather than picked out here. An idle callback is not
    # good enough: ttk dispatches its own selection event later than one of those, so a
    # link selected from one is wiped by the repaint that follows it.
    _pending_link_selection = link_elem

    tree = app_state.tree
    # This is also what records the visit, so Back returns to the sheet the link was
    # followed from.
    tree.selection_set(item_id)
    tree.focus(item_id)
    tree.see(item_id)


def open_link_from_link_view(link_elem):
    """Link View: leave for the wiresheet this link leads to, with the link selected.

    Where it leads is the sheet of the end that is not the component the view is about:
    a link that stays on one sheet goes back to that sheet, where both ends are drawn,
    and one that crosses to another sheet opens that other sheet -- which is the end the
    view could only show as a knob. A reference the document cannot resolve has no sheet
    of its own, so it falls back to the one the component itself is on.
    """
    focus = link_panel.comp if link_panel is not None else None
    root = app_state.xml_root_element
    if focus is None or root is None:
        return

    peer = next((component for component in link_endpoint_components(link_elem)
                 if component is not None and component is not focus), focus)
    sheet_element = find_parent_element(root, peer)

    # The sheet needs the pane back before it can be scrolled to the link: a hidden
    # canvas has no width to centre anything in.
    dismiss_link_view()
    go_to_sheet_with_link(sheet_element, link_elem)


def select_link_on_open_sheet(link_elem):
    """Selects a link on the sheet that has just been painted, and says which it is."""
    if select_link_element(link_elem):
        app_state.set_status(
            f"Link {link_elem.get('from', '?')} → {link_elem.get('to', '?')}"
        )
    else:
        app_state.set_status("That link is not painted on this sheet.", warn=True)


def delete_selected_component(event=None):
    """Deletes the highlighted canvas component (or selected link) and all its descendants recursively."""
    # Del is bound application-wide, so it also reaches a Properties field being typed in
    # and every other window. A keystroke aimed at a text field must not delete a
    # component, and neither must one pressed while the sheet is not even on screen.
    if editor_panel_showing():
        return

    focused = app_state.window.focus_get()
    if focused is not None:
        if focused.winfo_toplevel() is not app_state.window:
            return
        if focused.winfo_class() in TEXT_ENTRY_CLASSES:
            return

    if app_state.selected_link_tag:
        delete_selected_link()
        return

    if not app_state.selected_canvas_tag:
        return

    comp_element = app_state.canvas_comp_map.get(app_state.selected_canvas_tag)
    if comp_element is not None:
        delete_component(comp_element)


def on_tree_right_click(event):
    """Intercepts right-clicks on the treeview architecture to render branch context panels."""
    tree = app_state.tree
    item_id = tree.identify_row(event.y)
    if not item_id:
        return

    tree.selection_set(item_id)
    app_state.selected_tree_item_id = item_id
    parent_id = tree.parent(item_id)

    if parent_id == "":
        tree_root_context_menu.post(event.x_root, event.y_root)
        return

    comp_element = app_state.tree_element_map.get(item_id)
    if comp_element is None:
        return

    # The same actions the canvas offers, on the component the tree row names: which
    # list a component was picked from should not change what can be done with it.
    fill_component_popup(tree_comp_context_menu, comp_element)
    tree_comp_context_menu.post(event.x_root, event.y_root)


def on_canvas_click(event):
    canvas = app_state.canvas
    canvas.itemconfig("block_outline", outline="#7f9db9", width=2)
    wiresheet.deselect_link()
    app_state.drag_data["tag"] = None
    app_state.selected_canvas_tag = None

    cx = canvas.canvasx(event.x)
    cy = canvas.canvasy(event.y)
    clicked_item = canvas.find_closest(cx, cy, halo=3)

    if not clicked_item:
        return

    tags = canvas.gettags(clicked_item[0])
    group_tag = next((t for t in tags if t.startswith("comp_group_")), None)
    wire_tag = next((t for t in tags if t.startswith("link_wire_")), None)

    if wire_tag:
        wiresheet.select_link(wire_tag)
        return

    if group_tag:
        app_state.drag_data["tag"] = group_tag
        app_state.drag_data["x"] = cx
        app_state.drag_data["y"] = cy
        app_state.selected_canvas_tag = group_tag

        # Where the box starts out. The drag is measured as an offset from this pair
        # rather than accumulated per motion event, so repeated snapping cannot drift.
        origin = wiresheet.get_box_origin(group_tag)
        app_state.drag_data["box_x"] = origin[0] if origin else None
        app_state.drag_data["box_y"] = origin[1] if origin else None

        # Snapshot the pre-drag meta so the move can be pushed as one undoable step.
        dragged = app_state.canvas_comp_map.get(group_tag)
        meta_prop = dragged.find("prop[@name='meta']") if dragged is not None else None
        app_state.drag_data["meta_before"] = meta_prop.get("val") if meta_prop is not None else None
        canvas.itemconfig(f"{group_tag} && block_outline", outline="#1f4e79", width=3)

        # Bring the clicked box clear of any neighbour it happens to sit under.
        canvas.tag_raise(group_tag)
        wiresheet.redraw_links()


def set_link_mark(comp):
    """Marks a component as one end of a link still to be made."""
    if comp is None:
        return
    app_state.link_mark_element = comp
    app_state.set_status(
        f"Marked '{comp.get('name', '?')}' — right-click the other component to link it."
    )


def _slot_linkable(slot_flags, slot_type, readonly, is_target):
    """Returns False if a slot cannot participate in a link on the given side."""
    if is_target and readonly:
        return False
    if "a" in slot_flags or slot_type == "void":
        return False
    return True


def open_link_dialog(source_element, target_element):
    """Opens a dialog to select source and target slots, validates types, and creates a link."""
    window = app_state.window

    sides = [source_element, target_element]

    dialog = tk.Toplevel(window)
    dialog.title("Link")
    dialog.resizable(False, False)
    dialog.transient(window)
    dialog.grab_set()
    dialog.geometry(f"+{window.winfo_x() + 100}+{window.winfo_y() + 60}")

    main_frame = tk.Frame(dialog, padx=10, pady=10)
    main_frame.pack(fill=tk.BOTH, expand=True)

    src_frame = tk.LabelFrame(main_frame, padx=5, pady=5)
    src_frame.grid(row=0, column=0, sticky="nsew", padx=(0, 5))

    mid_frame = tk.Frame(main_frame)
    mid_frame.grid(row=0, column=1, padx=5)

    tgt_frame = tk.LabelFrame(main_frame, padx=5, pady=5)
    tgt_frame.grid(row=0, column=2, sticky="nsew", padx=(5, 0))

    src_var = tk.StringVar(value="")
    tgt_var = tk.StringVar(value="")

    src_widgets = []
    tgt_widgets = []
    src_slot_info = {}
    tgt_slot_info = {}

    status_var = tk.StringVar(value="")

    def _update_peer_states(selected_var, selected_info, peer_widgets, peer_info, peer_is_target):
        """Grey out incompatible slots on the peer side after a selection."""
        chosen = selected_var.get()
        if not chosen or chosen not in selected_info:
            for rb in peer_widgets:
                name = rb.cget("value")
                info = peer_info.get(name)
                if info and _slot_linkable(info[1], info[0], info[2], peer_is_target):
                    rb.config(state=tk.NORMAL, fg="black")
                else:
                    rb.config(state=tk.DISABLED, fg="#999999")
            return

        sel_type = selected_info[chosen][0]

        for rb in peer_widgets:
            name = rb.cget("value")
            info = peer_info.get(name)
            if not info:
                rb.config(state=tk.DISABLED, fg="#999999")
                continue

            peer_type, peer_flags, peer_ro = info
            if not _slot_linkable(peer_flags, peer_type, peer_ro, peer_is_target):
                rb.config(state=tk.DISABLED, fg="#999999")
            elif peer_type != sel_type:
                rb.config(state=tk.DISABLED, fg="#999999")
            else:
                rb.config(state=tk.NORMAL, fg="black")

    def _on_src_change(*_args):
        _update_peer_states(src_var, src_slot_info, tgt_widgets, tgt_slot_info, True)
        _update_status_label()

    def _on_tgt_change(*_args):
        _update_peer_states(tgt_var, tgt_slot_info, src_widgets, src_slot_info, False)
        _update_status_label()

    def _update_status_label():
        s = src_var.get()
        t = tgt_var.get()
        left_name = sides[0].get("name", "?")
        right_name = sides[1].get("name", "?")
        if s and t:
            status_var.set(f"Link {left_name}.{s} -> {right_name}.{t}")
        elif s:
            status_var.set(f"Link {left_name}.{s} -> ?")
        elif t:
            status_var.set(f"Link ? -> {right_name}.{t}")
        else:
            status_var.set("")

    def populate_panel(parent_frame, slots, var, widgets_list, info_dict, is_target):
        for child in parent_frame.winfo_children():
            child.destroy()
        widgets_list.clear()
        info_dict.clear()

        panel_canvas = tk.Canvas(parent_frame, width=160, height=300, highlightthickness=0)
        panel_scroll = ttk.Scrollbar(parent_frame, orient=tk.VERTICAL, command=panel_canvas.yview)
        inner = tk.Frame(panel_canvas)
        panel_canvas.create_window((0, 0), window=inner, anchor="nw")
        panel_canvas.config(yscrollcommand=panel_scroll.set)
        panel_canvas.pack(side=tk.LEFT, fill=tk.BOTH, expand=True)
        panel_scroll.pack(side=tk.RIGHT, fill=tk.Y)

        for slot_name, slot_type, slot_flags, slot_ro in slots:
            info_dict[slot_name] = (slot_type, slot_flags, slot_ro)
            linkable = _slot_linkable(slot_flags, slot_type, slot_ro, is_target)
            rb = tk.Radiobutton(inner, text=slot_name, variable=var, value=slot_name,
                                anchor="w", bg="#c8d4e8", relief="flat", padx=4, pady=1,
                                state=tk.NORMAL if linkable else tk.DISABLED,
                                fg="black" if linkable else "#999999",
                                disabledforeground="#999999")
            rb.pack(fill=tk.X)
            widgets_list.append(rb)

        inner.update_idletasks()
        panel_canvas.config(scrollregion=panel_canvas.bbox("all"))

    def refresh_panels():
        left = sides[0]
        right = sides[1]
        left_name = left.get("name", "?")
        right_name = right.get("name", "?")
        left_type = left.get("type", "")
        right_type = right.get("type", "")

        src_frame.config(text=f"{left_name} [Source]")
        tgt_frame.config(text=f"{right_name} [Target]")

        left_slots = schema.get_all_slots_for_type(left_type)
        right_slots = schema.get_all_slots_for_type(right_type)

        src_var.set("")
        tgt_var.set("")
        populate_panel(src_frame, left_slots, src_var, src_widgets, src_slot_info, False)
        populate_panel(tgt_frame, right_slots, tgt_var, tgt_widgets, tgt_slot_info, True)
        _update_status_label()

    src_var.trace_add("write", _on_src_change)
    tgt_var.trace_add("write", _on_tgt_change)

    def on_swap():
        sides[0], sides[1] = sides[1], sides[0]
        refresh_panels()

    ttk.Button(mid_frame, text="⇄", width=3, command=on_swap).pack(pady=5)

    refresh_panels()

    status_label = tk.Label(main_frame, textvariable=status_var, anchor="w", fg="#333333")
    status_label.grid(row=1, column=0, columnspan=3, sticky="w", pady=(8, 0))

    def on_ok():
        src_slot = src_var.get()
        tgt_slot = tgt_var.get()

        if not src_slot or not tgt_slot:
            messagebox.showwarning("Link", "Select a slot from both source and target.", parent=dialog)
            return

        st = src_slot_info.get(src_slot, ("?", ""))[0]
        tt = tgt_slot_info.get(tgt_slot, ("?", ""))[0]

        if st != tt:
            messagebox.showwarning(
                "Slot type mismatch",
                f"Cannot link slots of different types.\n\n"
                f"Source: {src_slot} ({st})\n"
                f"Target: {tgt_slot} ({tt})",
                parent=dialog
            )
            return

        src_path = get_component_path(sides[0], app_state.xml_root_element)
        tgt_path = get_component_path(sides[1], app_state.xml_root_element)

        from_ref = f"{src_path}.{src_slot}"
        to_ref = f"{tgt_path}.{tgt_slot}"

        dialog.destroy()
        sax_file.add_link(from_ref, to_ref)

    btn_frame = tk.Frame(main_frame)
    btn_frame.grid(row=2, column=0, columnspan=3, pady=(15, 0))

    ttk.Button(btn_frame, text="OK", width=10, command=on_ok).pack(side=tk.LEFT, padx=5)
    ttk.Button(btn_frame, text="Cancel", width=10, command=dialog.destroy).pack(side=tk.LEFT, padx=5)


def fill_component_popup(menu, comp):
    """Fills a popup with the actions that apply to one component.

    Shared by the canvas and the navigation tree, so a component offers the same
    actions wherever it was right-clicked. The entries close over the component rather
    than reading a selection back, which is what lets the two popups act on the thing
    that was actually clicked.
    """
    menu.delete(0, tk.END)
    menu.add_command(
        label="Properties",
        command=lambda: show_properties(comp),
    )
    menu.add_command(
        label="Link View",
        command=lambda: show_link_view(comp),
    )
    menu.add_separator()
    menu.add_command(
        label="Delete Component",
        command=lambda: delete_component(comp),
    )
    menu.add_separator()
    menu.add_command(label="Link Mark", command=lambda: set_link_mark(comp))

    marked = app_state.link_mark_element
    if marked is not None:
        mark_name = marked.get("name", "?")
        menu.add_command(
            label=f'Link From "{mark_name}"',
            command=lambda: open_link_dialog(marked, comp),
        )
        menu.add_command(
            label=f'Link To "{mark_name}"',
            command=lambda: open_link_dialog(comp, marked),
        )


def on_canvas_right_click(event):
    """Detects right-clicks on nodes to instantly show context-sensitive action overlays."""
    canvas = app_state.canvas
    cx = canvas.canvasx(event.x)
    cy = canvas.canvasy(event.y)
    clicked_item = canvas.find_closest(cx, cy, halo=3)

    if clicked_item:
        tags = canvas.gettags(clicked_item[0])
        group_tag = next((t for t in tags if t.startswith("comp_group_")), None)
        wire_tag = next((t for t in tags if t.startswith("link_wire_")), None)

        if wire_tag:
            wiresheet.select_link(wire_tag)
            _rebuild_link_context_menu()
            link_context_menu.post(event.x_root, event.y_root)
            return

        if group_tag:
            wiresheet.deselect_link()
            canvas.itemconfig("block_outline", outline="#7f9db9", width=2)
            app_state.selected_canvas_tag = group_tag
            canvas.itemconfig(f"{group_tag} && block_outline", outline="#1f4e79", width=3)

            comp_element = app_state.canvas_comp_map.get(group_tag)
            if comp_element is not None:
                fill_component_popup(canvas_context_menu, comp_element)
                canvas_context_menu.post(event.x_root, event.y_root)


def on_canvas_drag(event):
    if not app_state.drag_data["tag"]:
        return

    canvas = app_state.canvas
    group_tag = app_state.drag_data["tag"]
    cx = canvas.canvasx(event.x)
    cy = canvas.canvasy(event.y)

    box_x = app_state.drag_data.get("box_x")
    origin = wiresheet.get_box_origin(group_tag)

    if box_x is None or origin is None:
        # Nothing to align against, so the box just follows the pointer one to one.
        canvas.move(group_tag, cx - app_state.drag_data["x"], cy - app_state.drag_data["y"])
        app_state.drag_data["x"] = cx
        app_state.drag_data["y"] = cy
        wiresheet.redraw_links()
        return

    # Snap to whole meta steps, which is the grid on an ordinary sheet and a multiple of
    # it on one too wide for meta's range -- a position between steps cannot be stored.
    # Canvas coordinates carry the sheet's zoom, so the step and the margin do too.
    step = app_state.meta_scale * app_state.zoom
    margin = wiresheet.layout_margin(app_state.meta_scale) * app_state.zoom

    # Follow the pointer, but land the box on a step every step of the way, and keep it
    # out of the margin strip along the top and left canvas edges.
    wanted_x = box_x + (cx - app_state.drag_data["x"])
    wanted_y = app_state.drag_data["box_y"] + (cy - app_state.drag_data["y"])
    target_x = max(margin, round(wanted_x / step) * step)
    target_y = max(margin, round(wanted_y / step) * step)

    shift_x = target_x - origin[0]
    shift_y = target_y - origin[1]

    if shift_x or shift_y:
        canvas.move(group_tag, shift_x, shift_y)
        # Wires are not part of the dragged group, so they are re-routed against the
        # new positions.
        wiresheet.redraw_links()


def on_canvas_release(event):
    if not app_state.drag_data["tag"]:
        return

    canvas = app_state.canvas
    step = app_state.meta_scale * app_state.zoom
    group_tag = app_state.drag_data["tag"]
    origin = wiresheet.get_box_origin(group_tag)

    if origin is not None:
        current_x, current_y = origin

        margin = wiresheet.layout_margin(app_state.meta_scale) * app_state.zoom
        snapped_x = max(margin, round(current_x / step) * step)
        snapped_y = max(margin, round(current_y / step) * step)

        shift_x = snapped_x - current_x
        shift_y = snapped_y - current_y

        if shift_x != 0 or shift_y != 0:
            canvas.move(group_tag, shift_x, shift_y)
            app_state.mark_dirty()

        raw_x = min(wiresheet.META_COORD_MAX, int(round(snapped_x / step)))
        raw_y = min(wiresheet.META_COORD_MAX, int(round(snapped_y / step)))

        comp_element = app_state.canvas_comp_map.get(group_tag)
        if comp_element is not None:
            meta_str = None
            orig_prop = comp_element.find("prop[@name='meta']")
            if orig_prop is not None:
                meta_str = orig_prop.get("val")

            orig_x, orig_y, _, _, _, _, _, _ = read_meta_property(meta_str)
            if orig_x != raw_x or orig_y != raw_y:
                meta_before = app_state.drag_data.get("meta_before", meta_str)
                update_meta_property(comp_element, meta_str, raw_x, raw_y)
                undo.push(undo.MetaMoveCommand(
                    [(comp_element, meta_before, raw_x, raw_y)],
                    label=f"move {comp_element.get('name', 'component')}"
                ))
                app_state.mark_dirty()

    app_state.drag_data["tag"] = None
    wiresheet.redraw_links()


def fit_tree_column(tree):
    """Sizes a Treeview's tree column to its widest visible row.

    ttk derives the horizontal scroll range from the column width, so a column left to
    stretch with the widget can never scroll - long text is simply clipped. Sizing it to
    the content is what gives the horizontal scrollbar something to move, and the max()
    keeps a short list filling the width it has instead of leaving a bare strip.
    """
    if tree is None or not tree.winfo_exists():
        return

    style = ttk.Style()
    text_font = tkfont.nametofont(style.lookup("Treeview", "font") or "TkDefaultFont")
    try:
        indent = int(style.lookup("Treeview", "indent") or TREE_INDENT)
    except (ValueError, tk.TclError):
        indent = TREE_INDENT

    widest = 0

    def measure(node, depth):
        nonlocal widest
        for child in tree.get_children(node):
            width = depth * indent + text_font.measure(tree.item(child, "text"))
            widest = max(widest, width)
            # A closed branch hides its children, so they are not part of the width.
            if tree.item(child, "open"):
                measure(child, depth + 1)

    measure("", 1)
    tree.column("#0", width=max(widest + TREE_TEXT_PADDING, tree.winfo_width() - 2),
                minwidth=TREE_MIN_COLUMN_WIDTH, stretch=False)


def attach_tree_scrollbars(container, tree):
    """Puts a Treeview in its container with scrollbars that show only when they help.

    The tree/palette column is a fixed width now, so a deep path or a long type name has
    to be reachable by scrolling sideways rather than by widening the window. A bar that
    has nothing to scroll is taken out of the grid so it costs no room.
    """
    vertical = ttk.Scrollbar(container, orient=tk.VERTICAL, command=tree.yview)
    horizontal = ttk.Scrollbar(container, orient=tk.HORIZONTAL, command=tree.xview)
    vertical_cell = {"row": 0, "column": 1, "sticky": "ns"}
    horizontal_cell = {"row": 1, "column": 0, "sticky": "ew"}

    def show_when_needed(bar, cell, first, last):
        if float(first) <= 0.0 and float(last) >= 1.0:
            bar.grid_remove()
        else:
            bar.grid(**cell)
        bar.set(first, last)

    tree.configure(
        yscrollcommand=lambda first, last: show_when_needed(vertical, vertical_cell, first, last),
        xscrollcommand=lambda first, last: show_when_needed(horizontal, horizontal_cell, first, last),
    )

    container.rowconfigure(0, weight=1)
    container.columnconfigure(0, weight=1)
    tree.grid(row=0, column=0, sticky="nsew")

    # Resizing, and opening or closing a branch, both change what the widest row is.
    tree.bind("<Configure>", lambda event: fit_tree_column(tree), add="+")
    def _on_tree_open(event):
        tree.after_idle(fit_tree_column, tree)
        if _restoring_selection:
            return
        if app_state.live_mode:
            opened = tree.focus()
            if opened:
                elem = app_state.tree_element_map.get(opened)
                if elem is not None:
                    cid = elem.get("id")
                    if cid is not None:
                        _live_tree_expand(int(cid), opened)

    tree.bind("<<TreeviewOpen>>", _on_tree_open, add="+")
    tree.bind("<<TreeviewClose>>", lambda event: tree.after_idle(fit_tree_column, tree), add="+")


def clamp_left_pane_split(event=None):
    """Keeps both halves of the left column visible, whatever the sash or the window does.

    ttk lets a sash drag run to either end, and it remembers that position when the window
    is made shorter afterwards, so either list can be left with no height at all. Each
    pane's requested height is the line. Once the column is too short to seat both, the
    floors are dropped and ttk's own proportional squeeze takes over.
    """
    total = left_pane.winfo_height()
    tree_floor = frame_tree.winfo_reqheight()
    palette_floor = frame_palette.winfo_reqheight()
    if total <= tree_floor + palette_floor:
        return

    position = left_pane.sashpos(0)
    clamped = min(max(position, tree_floor), total - palette_floor)
    if clamped != position:
        left_pane.sashpos(0, clamped)


def apply_left_pane_width():
    """Pins the tree/palette column to the configured width.

    Setting the sash before the paned window has been laid out collapses the column
    instead of sizing it, so the startup call retries until there is a width to divide.
    This is also how a changed preference reaches an already open window.
    """
    app_state.window.update_idletasks()
    if paned_window.winfo_width() <= 1:
        paned_window.after(50, apply_left_pane_width)
        return
    paned_window.sashpos(0, app_state.LEFT_PANE_WIDTH)


def apply_left_column_visibility():
    """Shows or hides the tree/palette column to match the View menu's checkbutton.

    ttk has no hide flag for a pane: the child is dropped from the paned window and put
    back at the front, which is why the configured width has to be re-applied afterwards.
    """
    if show_left_column_var is None:
        return

    panes = paned_window.panes()
    is_shown = str(left_pane) in [str(pane) for pane in panes]

    if show_left_column_var.get():
        if not is_shown:
            paned_window.insert(0, left_pane, weight=0)
            apply_left_pane_width()
    elif is_shown:
        paned_window.forget(left_pane)

    _sync_pane_toggle_button()


def toggle_left_column(event=None):
    """Keyboard route to the same toggle. The menu flips the variable for itself."""
    if show_left_column_var is not None:
        show_left_column_var.set(not show_left_column_var.get())
        apply_left_column_visibility()
    return "break"


def _sync_pane_toggle_button():
    """Update the toggle button icon to reflect the pane state."""
    if btn_toggle_pane is not None and show_left_column_var is not None:
        btn_toggle_pane.configure(text="◧" if show_left_column_var.get() else "▯")


# --- Sheet view history ---

def _tree_item_for_element(element):
    """The tree item whose sheet is this element's children, or None if it is gone.

    A history entry recorded before a delete, or before another file was opened, no
    longer has a node to select; that is what None reports.
    """
    tree = app_state.tree
    if tree is None:
        return None
    if element is None:
        # The file's root node, which parse never puts in the element map.
        children = tree.get_children()
        return children[0] if children else None
    for item_id, mapped in app_state.tree_element_map.items():
        if mapped is element:
            return item_id
    return None


def _entry_is_reachable(entry):
    """Whether a history entry still names something that could be shown.

    A sheet entry needs a tree node to select, which is what paints it. A panel entry
    needs only its component, since a panel takes the whole pane and is built straight
    from the document.
    """
    kind, element = entry
    if kind == VIEW_SHEET:
        return _tree_item_for_element(element) is not None
    return _component_in_document(element)


def _prune_view_history():
    """Drops entries that can no longer be shown, keeping the position on screen."""
    global history_index

    kept = []
    new_index = -1
    for position, entry in enumerate(view_history):
        if not _entry_is_reachable(entry):
            continue

        # Dropping an entry can leave the two either side of it naming the same view,
        # and a Back that visibly does nothing is worse than a shorter trail. The
        # position still lands on whichever of the two it was on, since they are the
        # same view either way.
        if kept and kept[-1][0] == entry[0] and kept[-1][1] is entry[1]:
            if position <= history_index:
                new_index = len(kept) - 1
            continue

        kept.append(entry)
        if position <= history_index:
            new_index = len(kept) - 1

    view_history[:] = kept
    history_index = min(new_index, len(kept) - 1)


def update_history_controls():
    """Greys out Back and Forward when there is nothing to walk to in that direction."""
    can_back = history_index > 0
    can_forward = 0 <= history_index < len(view_history) - 1

    for button, enabled in ((btn_back, can_back), (btn_forward, can_forward)):
        if button is not None:
            button.config(state="normal" if enabled else "disabled")

    if menu_view is not None:
        menu_view.entryconfig(
            MENU_BACK_LABEL, state="normal" if can_back else "disabled"
        )
        menu_view.entryconfig(
            MENU_FORWARD_LABEL, state="normal" if can_forward else "disabled"
        )


def reset_view_history():
    """Starts the history over, for a newly opened file."""
    global history_index
    view_history.clear()
    history_index = -1
    update_history_controls()


def record_visit(kind, element):
    """Adds a freshly opened view to the history, dropping anything ahead of it.

    Every way of filling the editor pane comes through here: the tree selection handler
    for a sheet, and the two panels as they are put up. Back and Forward suppress it,
    because walking the history must not rewrite the trail it is following.
    """
    global history_index, _history_file_path

    if _suppress_history:
        return

    # Entries name elements of one document, so a different file starts a fresh history
    # whether or not the tree rebuild that came with it was announced.
    if app_state.current_file_path != _history_file_path:
        _history_file_path = app_state.current_file_path
        view_history.clear()
        history_index = -1

    # Opening what is already on screen is not a move. Compared by identity, since two
    # sheets are the same sheet only when they are the same element.
    if 0 <= history_index < len(view_history):
        current_kind, current_element = view_history[history_index]
        if current_kind == kind and current_element is element:
            return

    del view_history[history_index + 1:]
    view_history.append((kind, element))
    if len(view_history) > HISTORY_LIMIT:
        del view_history[0]
    history_index = len(view_history) - 1
    update_history_controls()


def _show_history_entry(position):
    """Puts back what the pane held at this position, without recording the move.

    The pane is free by the time this runs: both ways in ask about unsaved edits and
    take whatever panel is up back down first.
    """
    global history_index, _suppress_history

    kind, element = view_history[position]
    if not _entry_is_reachable((kind, element)):
        return

    _suppress_history = True
    try:
        if kind == VIEW_SHEET:
            _show_sheet_entry(element)
        elif kind == VIEW_PROPERTIES:
            show_properties(element)
        else:
            show_link_view(element)
    finally:
        _suppress_history = False

    history_index = position
    update_history_controls()


def _show_sheet_entry(element):
    """Selects the tree node whose children are this sheet, which is what paints it."""
    tree = app_state.tree
    item_id = _tree_item_for_element(element)
    was_selected = tree.selection() == (item_id,)

    tree.selection_set(item_id)
    tree.focus(item_id)
    tree.see(item_id)

    if was_selected:
        # ttk fires no selection event when the node is already the selected one, so
        # the repaint this entry is asking for has to be made here instead. Leaving a
        # panel lands here every time: closing it puts the tree's own sheet back on
        # screen without the selection ever having moved.
        app_state.current_sheet_parent = element
        wiresheet.render_current_sheet()


def go_back(event=None):
    """View menu and toolbar: step to whatever the editor pane held before this."""
    if not close_editor_panel():
        return "break"
    _prune_view_history()
    if history_index > 0:
        _show_history_entry(history_index - 1)
    else:
        update_history_controls()
    return "break"


def go_forward(event=None):
    """View menu and toolbar: step back towards the most recently opened view."""
    if not close_editor_panel():
        return "break"
    _prune_view_history()
    if 0 <= history_index < len(view_history) - 1:
        _show_history_entry(history_index + 1)
    else:
        update_history_controls()
    return "break"


def on_tree_filled():
    """Runs after every rebuild of the navigation tree.

    A rebuild is either a new file, which retires the history, or a repaint after an
    undo or a delete, which can leave entries pointing at elements that are no longer
    in the document.
    """
    global _history_file_path

    # A different file has no such component, and a rebuild after a delete may have taken
    # it. Neither is a moment to ask about edits that now have nowhere to go.
    panel = properties_panel or link_panel
    if panel is not None and not _component_in_document(panel.comp):
        close_editor_panel(force=True)

    # The root row is new after every rebuild, so its colour has to be reapplied.
    refresh_dirty_display()
    fit_tree_column(app_state.tree)
    refresh_palette_kits()

    if app_state.current_file_path != _history_file_path:
        _history_file_path = app_state.current_file_path
        reset_view_history()

        if _schema_has_incomplete_checksums():
            show_kits()
    else:
        _prune_view_history()
        update_history_controls()


def _schema_has_incomplete_checksums():
    """Whether any declared kit is missing a checksum."""
    if not app_state.active_schema_kits:
        return False
    return any(cs is None for cs in app_state.active_schema_kits.values())


# --- Dialogs ---

def open_preferences_dialog():
    """Pops open a tabular modal dialog to edit configuration attributes."""
    window = app_state.window

    dialog = tk.Toplevel(window)
    dialog.title("Preferences")
    dialog.geometry("470x330")
    dialog.resizable(False, False)
    dialog.transient(window)
    dialog.grab_set()

    dialog.geometry(f"+{window.winfo_x() + 100}+{window.winfo_y() + 100}")

    frame_padded = ttk.Frame(dialog, padding=15)
    frame_padded.pack(fill=tk.BOTH, expand=True)

    frame_padded.columnconfigure(0, weight=1, minsize=160)
    frame_padded.columnconfigure(1, weight=2)

    lbl_home = ttk.Label(frame_padded, text="Sedona home folder:", anchor="w")
    lbl_home.grid(row=0, column=0, sticky="w", pady=(0, 10), padx=(0, 10))

    frame_home_picker = ttk.Frame(frame_padded)
    frame_home_picker.grid(row=0, column=1, sticky="ew", pady=(0, 10))
    frame_home_picker.columnconfigure(0, weight=1)

    entry_home = ttk.Entry(frame_home_picker)
    entry_home.insert(0, app_state.SEDONA_HOME)
    entry_home.grid(row=0, column=0, sticky="ew", padx=(0, 5))

    def browse_home_folder():
        chosen_dir = filedialog.askdirectory(title="Select Sedona Home Folder", initialdir=entry_home.get() or None)
        if chosen_dir:
            entry_home.delete(0, tk.END)
            entry_home.insert(0, os.path.normpath(chosen_dir))

    btn_home_browse = ttk.Button(frame_home_picker, text="...", width=3, command=browse_home_folder)
    btn_home_browse.grid(row=0, column=1, sticky="e")

    lbl_size = ttk.Label(frame_padded, text="Grid size (pixels):", anchor="w")
    lbl_size.grid(row=1, column=0, sticky="w", pady=(0, 10), padx=(0, 10))

    entry_grid = ttk.Entry(frame_padded)
    entry_grid.insert(0, str(app_state.GRID_SIZE))
    entry_grid.grid(row=1, column=1, sticky="ew", pady=(0, 10))

    lbl_box = ttk.Label(frame_padded, text="Component box width (pixels):", anchor="w")
    lbl_box.grid(row=2, column=0, sticky="w", pady=(0, 10), padx=(0, 10))

    entry_box_width = ttk.Entry(frame_padded)
    entry_box_width.insert(0, str(app_state.BOX_WIDTH))
    entry_box_width.grid(row=2, column=1, sticky="ew", pady=(0, 10))

    lbl_left_pane = ttk.Label(frame_padded, text="Tree/palette width (pixels):", anchor="w")
    lbl_left_pane.grid(row=3, column=0, sticky="w", pady=(0, 10), padx=(0, 10))

    entry_left_pane = ttk.Entry(frame_padded)
    entry_left_pane.insert(0, str(app_state.LEFT_PANE_WIDTH))
    entry_left_pane.grid(row=3, column=1, sticky="ew", pady=(0, 10))

    lbl_timestamp = ttk.Label(frame_padded, text="Save with Date-Time stamp?", anchor="w")
    lbl_timestamp.grid(row=4, column=0, sticky="w", pady=(0, 15), padx=(0, 10))

    var_timestamp = tk.BooleanVar(value=app_state.ADD_TIMESTAMP)
    chk_timestamp = ttk.Checkbutton(
        frame_padded,
        text="Yes",
        variable=var_timestamp
    )
    chk_timestamp.grid(row=4, column=1, sticky="w", pady=(0, 10))

    lbl_trace = ttk.Label(frame_padded, text="Trace DASP to terminal?", anchor="w")
    lbl_trace.grid(row=5, column=0, sticky="w", pady=(0, 15), padx=(0, 10))

    var_trace_dasp = tk.BooleanVar(value=app_state.TRACE_DASP)
    chk_trace_dasp = ttk.Checkbutton(
        frame_padded,
        text="Yes",
        variable=var_trace_dasp
    )
    chk_trace_dasp.grid(row=5, column=1, sticky="w", pady=(0, 15))

    def apply_preferences():
        try:
            val = int(entry_grid.get().strip())
            if val < 2 or val > 200:
                raise ValueError("Grid size must be a whole number between 2 and 200.")

            try:
                width = int(entry_box_width.get().strip())
            except ValueError:
                raise ValueError(
                    f"Box width must be a whole number between "
                    f"{app_state.MIN_ALLOWED_BOX_WIDTH} and {app_state.MAX_ALLOWED_BOX_WIDTH}."
                )
            if not (app_state.MIN_ALLOWED_BOX_WIDTH <= width <= app_state.MAX_ALLOWED_BOX_WIDTH):
                raise ValueError(
                    f"Box width must be between {app_state.MIN_ALLOWED_BOX_WIDTH} and "
                    f"{app_state.MAX_ALLOWED_BOX_WIDTH} pixels."
                )

            try:
                pane_width = int(entry_left_pane.get().strip())
            except ValueError:
                raise ValueError(
                    f"Tree/palette width must be a whole number between "
                    f"{app_state.MIN_ALLOWED_LEFT_PANE_WIDTH} and "
                    f"{app_state.MAX_ALLOWED_LEFT_PANE_WIDTH}."
                )
            if not (app_state.MIN_ALLOWED_LEFT_PANE_WIDTH <= pane_width
                    <= app_state.MAX_ALLOWED_LEFT_PANE_WIDTH):
                raise ValueError(
                    f"Tree/palette width must be between "
                    f"{app_state.MIN_ALLOWED_LEFT_PANE_WIDTH} and "
                    f"{app_state.MAX_ALLOWED_LEFT_PANE_WIDTH} pixels."
                )

            # The width cap is applied after each box is fitted to its text, so a cap
            # that is not a multiple of the grid leaves the widest boxes off-grid.
            proposed = wiresheet.suggest_box_width_for_grid(width, val)
            if proposed != width:
                if not messagebox.askyesno(
                    "Box Width Off Grid",
                    f"A box width of {width} px is not a multiple of the {val} px grid, "
                    f"so the widest boxes would not line up with it.\n\n"
                    f"Change the box width to {proposed} px?",
                    parent=dialog,
                ):
                    return
                width = proposed
                entry_box_width.delete(0, tk.END)
                entry_box_width.insert(0, str(width))

            app_state.BOX_WIDTH = width
            app_state.LEFT_PANE_WIDTH = pane_width
            app_state.GRID_SIZE = val
            app_state.ADD_TIMESTAMP = var_timestamp.get()
            app_state.TRACE_DASP = var_trace_dasp.get()
            app_state.SEDONA_HOME = entry_home.get().strip()

            app_state.save_preferences()

            apply_left_pane_width()
            wiresheet.draw_grid()
            if app_state.active_components_list:
                wiresheet.render_components(app_state.active_components_list)

            # Sedona home may have moved, so the palette's kit list is rebuilt.
            refresh_palette_kits()

            dialog.destroy()
        except ValueError as error:
            message = str(error) or "Please enter a valid integer size between 2 and 200."
            messagebox.showerror("Invalid Input", message, parent=dialog)

    btn_save_pref = ttk.Button(frame_padded, text="Apply Changes", command=apply_preferences)
    btn_save_pref.grid(row=6, column=1, sticky="e")

    if not app_state.SEDONA_HOME:
        entry_home.focus_set()
    else:
        entry_grid.focus_set()


def show_app_info():
    """Displays information tracking details for the currently active schema file in standard columns."""
    window = app_state.window

    if app_state.current_file_path:
        file_name = os.path.basename(app_state.current_file_path)
        folder_path = os.path.dirname(app_state.current_file_path)
    else:
        file_name = "No profile loaded"
        folder_path = "None"

    lastReachedID = 0
    all_found_ids = set()

    if app_state.xml_root_element is not None:
        for elem in app_state.xml_root_element.iter():
            if elem.tag == "comp":
                id_str = elem.get("id")
                if id_str is not None:
                    try:
                        val = int(id_str)
                        all_found_ids.add(val)
                        if val > lastReachedID:
                            lastReachedID = val
                    except ValueError:
                        pass

    not_used_ids = [i for i in range(1, lastReachedID + 1) if i not in all_found_ids]
    usedComponentNumber = lastReachedID - len(not_used_ids)

    not_used_str = ", ".join(map(str, not_used_ids)) if not_used_ids else "None"
    if len(not_used_str) > 40:
        not_used_str = not_used_str[:37] + "..."

    info_win = tk.Toplevel(window)
    info_win.title("App Info")
    info_win.resizable(False, False)
    info_win.transient(window)
    info_win.grab_set()

    frame = ttk.Frame(info_win, padding=15)
    frame.pack(fill=tk.BOTH, expand=True)

    rows_data = [
        ("App File Name:", file_name),
        ("File Location:", folder_path),
        ("Last Reached ID:", str(lastReachedID)),
        ("Used Component Count:", str(max(0, usedComponentNumber))),
        ("Unused IDs:", not_used_str)
    ]

    for idx, (lbl_txt, val_txt) in enumerate(rows_data):
        lbl = tk.Label(frame, text=lbl_txt, font=("TkDefaultFont", 9, "bold"), anchor="w")
        lbl.grid(row=idx, column=0, sticky="w", padx=(0, 15), pady=4)

        val = tk.Label(frame, text=val_txt, anchor="w", justify="left")
        val.grid(row=idx, column=1, sticky="w", pady=4)

    info_win.update_idletasks()
    x = window.winfo_x() + (window.winfo_width() // 2) - (info_win.winfo_width() // 2)
    y = window.winfo_y() + (window.winfo_height() // 2) - (info_win.winfo_height() // 2)
    info_win.geometry(f"+{x}+{y}")


def verify_startup_configurations():
    """Checks for configuration gaps, triggers an alert, and opens setup if empty."""
    if not app_state.SEDONA_HOME:
        messagebox.showwarning(
            "Configuration Required",
            "The 'Sedona home folder' path is not configured.\n\n"
            "Please specify the directory path in the configuration options "
            "before trying to view or load profile source files."
        )
        open_preferences_dialog()
        return

    # editor.properties can be hand-edited, so the pair is reconciled here too.
    proposed = wiresheet.suggest_box_width_for_grid(app_state.BOX_WIDTH, app_state.GRID_SIZE)
    if proposed != app_state.BOX_WIDTH and messagebox.askyesno(
        "Box Width Off Grid",
        f"The saved box width of {app_state.BOX_WIDTH} px is not a multiple of the "
        f"{app_state.GRID_SIZE} px grid, so the widest boxes will not line up with it.\n\n"
        f"Change the box width to {proposed} px?",
    ):
        app_state.BOX_WIDTH = proposed
        app_state.save_preferences()
        wiresheet.draw_grid()
        if app_state.active_components_list:
            wiresheet.render_components(app_state.active_components_list)


# ── Live device connection ──────────────────────────────────────

_cov_poll_id = None


def do_device_connect():
    """Opens the connect dialog and initiates a device connection."""
    if app_state.current_file_path and not app_state.live_mode:
        if app_state.is_dirty:
            answer = messagebox.askyesnocancel(
                "Unsaved Changes",
                "The current file has unsaved changes.\n\n"
                "Save before connecting to a device?",
            )
            if answer is None:
                return
            if answer:
                sax_file.execute_direct_overwrite_save()
        sax_file.reset_to_startup_state()
        sax_file.refresh_treeview_from_dom(None)
        wiresheet.render_components([])
        app_state.window.title(app_state.base_window_title())

    connect_dialog.ConnectDialog(app_state.window, _connect_to_device)


def do_device_discover():
    """Opens the discover dialog to scan for devices on the network."""
    if app_state.current_file_path and not app_state.live_mode:
        if app_state.is_dirty:
            answer = messagebox.askyesnocancel(
                "Unsaved Changes",
                "The current file has unsaved changes.\n\n"
                "Save before connecting to a device?",
            )
            if answer is None:
                return
            if answer:
                sax_file.execute_direct_overwrite_save()
        sax_file.reset_to_startup_state()
        sax_file.refresh_treeview_from_dom(None)
        wiresheet.render_components([])
        app_state.window.title(app_state.base_window_title())

    connect_dialog.DiscoverDialog(app_state.window, _connect_to_device)


def _connect_to_device(host, port, user, password):
    """Called from the connect dialog's background thread."""
    ds = device_session.DeviceSession()
    root_comp = ds.connect(host, port, user, password)

    app_state.device_session = ds
    app_state.device_host = host
    app_state.device_port = port
    app_state.live_mode = True
    app_state.connection_status = "connected"

    app_state.window.after(0, lambda: _on_connected(ds, root_comp))
    return root_comp


def _on_connected(ds, root_comp):
    """Main-thread callback after successful connect."""
    app_state.window.title(app_state.base_window_title_live())
    menu_device.entryconfig("Connect...", state="disabled")
    menu_device.entryconfig("Discover...", state="disabled")
    menu_device.entryconfig("Disconnect", state="normal")
    menu_device.entryconfig("Download to SAX...", state="normal")
    app_state.set_status(f"Connected to {app_state.device_host}:{app_state.device_port}")

    sax_root = ds.build_dom_tree(root_comp)
    app_state.xml_root_element = sax_root
    app_state.current_file_path = f"device://{app_state.device_host}"

    _load_live_schema(ds)
    sax_file.refresh_treeview_from_dom(sax_root)

    undo.reset()
    app_state.mark_clean()
    _start_cov_polling()


def _load_live_schema(ds):
    """Loads schema kits from the device's schema parts into the editor registry."""
    app_state.active_schema_kits.clear()
    app_state.final_components_registry.clear()
    if not (ds.sox_client and ds.sox_client.schema_parts):
        return

    for part in ds.sox_client.schema_parts:
        app_state.active_schema_kits[part.name] = f"{part.checksum & 0xFFFFFFFF:08x}"

    # Collect manifests we have, skip missing kits (device may use kits
    # not installed on this computer — load what we can)
    manifest_paths = []
    missing = []
    for name, checksum in app_state.active_schema_kits.items():
        path = schema.find_manifest_path(name, checksum)
        if path:
            manifest_paths.append(path)
        else:
            missing.append(name)

    if manifest_paths:
        schema.compile_schema_registry(manifest_paths)

    if missing:
        app_state.set_status(
            f"Missing kit manifests: {', '.join(missing)}", warn=True)


def do_device_disconnect():
    """Disconnects from the live device."""
    global _cov_poll_id

    if _cov_poll_id is not None:
        app_state.window.after_cancel(_cov_poll_id)
        _cov_poll_id = None

    ds = app_state.device_session
    if ds is not None:
        ds.disconnect()

    app_state.device_session = None
    app_state.live_mode = False
    app_state.connection_status = "offline"
    app_state.device_host = ""
    app_state.device_port = 1876

    app_state.window.title(app_state.base_window_title())
    menu_device.entryconfig("Connect...", state="normal")
    menu_device.entryconfig("Discover...", state="normal")
    menu_device.entryconfig("Disconnect", state="disabled")
    menu_device.entryconfig("Download to SAX...", state="disabled")
    app_state.set_status("Disconnected")

    sax_file.reset_to_startup_state()
    sax_file.refresh_treeview_from_dom(None)
    wiresheet.render_components([])


def _start_cov_polling():
    """Starts the 100ms COV event polling loop on the main thread."""
    global _cov_poll_id

    def poll():
        global _cov_poll_id
        ds = app_state.device_session
        if ds is None or not ds.is_connected:
            _cov_poll_id = None
            if app_state.live_mode:
                app_state.connection_status = "error"
                app_state.set_status("Connection lost", warn=True)
            return

        events = ds.poll_events()
        if events and app_state.xml_root_element is not None:
            needs_repaint = False
            for comp_id, what_char in events:
                elem = ds.update_element_from_cache(comp_id, app_state.xml_root_element)
                if elem is not None:
                    needs_repaint = True
            if needs_repaint:
                wiresheet.render_current_sheet()
                if properties_panel is not None:
                    try:
                        properties_panel.draw()
                    except Exception:
                        pass

        _cov_poll_id = app_state.window.after(100, poll)

    _cov_poll_id = app_state.window.after(100, poll)


def _live_tree_expand(comp_id, tree_item_id):
    """Called when user expands a tree node in live mode to lazy-load children."""
    ds = app_state.device_session
    if ds is None or not ds.is_connected:
        return

    app_state.set_status("Loading children...")

    def _bg():
        try:
            children = ds.load_children(comp_id)
            if not children:
                app_state.window.after(0, lambda: app_state.set_status("No children"))
                return
            sax_root = ds.build_dom_tree(ds.sox_client.cache_get(0))
            ds.build_links_elements(sax_root)

            def _ui():
                global _restoring_selection
                app_state.xml_root_element = sax_root
                sax_file.refresh_treeview_from_dom(sax_root)
                # Re-open and re-select the node without triggering a wiresheet
                # re-render — the sheet is already showing the right content and
                # a redundant close_wiresheet + open_wiresheet cycle can lose links.
                for iid, elem in app_state.tree_element_map.items():
                    if elem.get("id") == str(comp_id):
                        app_state.current_sheet_parent = elem
                        _restoring_selection = True
                        tree.item(iid, open=True)
                        tree.selection_set(iid)
                        tree.see(iid)
                        tree.after_idle(_clear_restoring_flag)
                        break
                app_state.set_status(f"Loaded {len(children)} children")

            app_state.window.after(0, _ui)
        except Exception as exc:
            app_state.window.after(
                0, lambda: app_state.set_status(f"Load error: {exc}", warn=True)
            )

    threading.Thread(target=_bg, daemon=True).start()


# --- UI Setup ---
window = tk.Tk()
app_state.window = window
window.title(app_state.base_window_title())
window.geometry("950x600")
# Width is left alone; only the height needs a floor, to keep the tree pane off zero.
window.minsize(1, MIN_WINDOW_HEIGHT)

# --- Context Popups Structures ---
canvas_context_menu = tk.Menu(window, tearoff=0)
link_context_menu = tk.Menu(window, tearoff=0)


def _rebuild_link_context_menu():
    """Rebuilds the link popup, naming the sheet the far end of the link is on.

    A link with both ends on the open sheet is already fully in view, so the jump is
    left off rather than greyed out.
    """
    link_context_menu.delete(0, tk.END)
    link_context_menu.add_command(label="Delete Link", command=delete_selected_link)

    link_elem = app_state.canvas_link_map.get(app_state.selected_link_tag)
    destination = link_target_sheet(link_elem) if link_elem is not None else None
    if destination is None:
        return

    sheet_name = link_view.sheet_display_name(destination[0])

    link_context_menu.add_separator()
    link_context_menu.add_command(
        label=f'Go to Link Wire Sheet "{sheet_name}"', command=go_to_link_sheet
    )


tree_comp_context_menu = tk.Menu(window, tearoff=0)

tree_root_context_menu = tk.Menu(window, tearoff=0)
tree_root_context_menu.add_command(label="Save", command=sax_file.execute_direct_overwrite_save)

# --- Keyboard shortcuts ---
# Command and Control are both bound on every platform, so muscle memory from either
# side works and Windows users are not asked to learn a Mac chord.
SHORTCUTS = [
    ("s", do_save),
    ("z", do_undo),
    ("y", do_redo),
    ("b", toggle_left_column),
    # Zoom, in every spelling the key can arrive as: with and without Shift, and on the
    # numeric keypad.
    ("plus", do_zoom_in),
    ("equal", do_zoom_in),
    ("KP_Add", do_zoom_in),
    ("minus", do_zoom_out),
    ("underscore", do_zoom_out),
    ("KP_Subtract", do_zoom_out),
    ("0", do_zoom_reset),
    ("KP_0", do_zoom_reset),
    ("9", do_zoom_fit),
    ("KP_9", do_zoom_fit),
    ("p", wiresheet.print_sheet),
]
for key, handler in SHORTCUTS:
    window.bind_all(f"<Command-{key}>", handler)
    window.bind_all(f"<Control-{key}>", handler)

# Redo also as Shift-Z, the usual mac chord.
window.bind_all("<Command-Shift-Z>", do_redo)
window.bind_all("<Control-Shift-Z>", do_redo)

# Delete: the mac Delete key reports as BackSpace, the Windows one as Delete.
window.bind_all("<Delete>", delete_selected_component)
window.bind_all("<BackSpace>", delete_selected_component)

# Tidy Layout is the one shortcut bound on Control alone: Cmd+W closes the window on
# macOS, and shadowing that would be a poor trade for a second way to re-flow a sheet.
window.bind_all("<Control-w>", do_tidy_layout)

# Sheet history: the browser chords, in both the Alt and the mac bracket spelling.
window.bind_all("<Alt-Left>", go_back)
window.bind_all("<Alt-Right>", go_forward)
window.bind_all("<Command-bracketleft>", go_back)
window.bind_all("<Command-bracketright>", go_forward)

accel_mod = "Cmd" if sys.platform == "darwin" else "Ctrl"

# --- Native Application Menu Configurations ---
menu_bar = tk.Menu(window)

menu_device = tk.Menu(menu_bar, tearoff=0)
menu_device.add_command(label="Connect...", command=do_device_connect)
menu_device.add_command(label="Discover...", command=do_device_discover)
menu_device.add_command(label="Disconnect", command=do_device_disconnect, state="disabled")
menu_device.add_separator()
menu_device.add_command(label="Download to SAX...", command=do_download_to_sax, state="disabled")
menu_bar.add_cascade(label="Device", menu=menu_device)

menu_file = tk.Menu(menu_bar, tearoff=0)
menu_file.add_command(label="Open", command=sax_file.open_file)
menu_file.add_command(label="Save", command=do_save, accelerator=f"{accel_mod}+S")
menu_file.add_command(label="Save As...", command=sax_file.save_file_as)
menu_file.add_separator()
menu_file.add_command(label="Print...", command=wiresheet.print_sheet, accelerator=f"{accel_mod}+P")
menu_file.add_separator()
menu_file.add_command(label="Exit", command=window.quit)
menu_bar.add_cascade(label="File", menu=menu_file)

menu_edit = tk.Menu(menu_bar, tearoff=0)
menu_edit.add_command(label="Undo", command=do_undo, accelerator=f"{accel_mod}+Z")
menu_edit.add_command(label="Redo", command=do_redo, accelerator=f"{accel_mod}+Y")
menu_edit.add_separator()
menu_edit.add_command(label="Delete Component", command=delete_selected_component, accelerator="Del")
menu_bar.add_cascade(label="Edit", menu=menu_edit)

menu_view = tk.Menu(menu_bar, tearoff=0)
show_left_column_var = tk.BooleanVar(value=True)
menu_view.add_checkbutton(
    label="Tree / Palette",
    variable=show_left_column_var,
    command=apply_left_column_visibility,
    accelerator=f"{accel_mod}+B",
)
menu_view.add_separator()
menu_view.add_command(
    label=MENU_BACK_LABEL, command=go_back, accelerator="Alt+Left", state="disabled"
)
menu_view.add_command(
    label=MENU_FORWARD_LABEL, command=go_forward, accelerator="Alt+Right", state="disabled"
)
menu_view.add_separator()
menu_view.add_command(label="Zoom In", command=do_zoom_in, accelerator=f"{accel_mod}++")
menu_view.add_command(label="Zoom Out", command=do_zoom_out, accelerator=f"{accel_mod}+-")
menu_view.add_command(label="Zoom 100%", command=do_zoom_reset, accelerator=f"{accel_mod}+0")
menu_view.add_command(label="Zoom to Fit", command=do_zoom_fit, accelerator=f"{accel_mod}+9")
menu_view.add_separator()
menu_view.add_command(label="Tidy Layout", command=do_tidy_layout, accelerator="Ctrl+W")
menu_bar.add_cascade(label="View", menu=menu_view)

menu_kits = tk.Menu(menu_bar, tearoff=0)
menu_kits.add_command(
    label="Manage Kits...",
    command=show_kits
)
menu_bar.add_cascade(label="Kits", menu=menu_kits)

menu_app_info = tk.Menu(menu_bar, tearoff=0)
menu_app_info.add_command(label="App Info", command=show_app_info)
menu_bar.add_cascade(label="App", menu=menu_app_info)

menu_prefs = tk.Menu(menu_bar, tearoff=0)
menu_prefs.add_command(label="Preferences", command=open_preferences_dialog)
menu_bar.add_cascade(label="Preferences", menu=menu_prefs)

window.config(menu=menu_bar)

# Packed before the paned window: pack hands out the cavity in order, and the paned
# window expands into whatever is left, so a status bar added afterwards would get no room.
status_label = ttk.Label(window, text="", anchor="w", foreground=app_state.STATUS_INFO_COLOUR)
status_label.pack(side=tk.BOTTOM, fill=tk.X, padx=8, pady=(0, 4))
app_state.status_label = status_label

# Packed ahead of the paned window for the same reason the status bar is.
toolbar = ttk.Frame(window)
toolbar.pack(side=tk.TOP, fill=tk.X, padx=8, pady=(4, 0))

# U+25E7: a square with left half filled — suggests a visible side panel.
btn_toggle_pane = ttk.Button(toolbar, text="◧", width=3, command=toggle_left_column)
btn_toggle_pane.pack(side=tk.LEFT)
_ToolTip(btn_toggle_pane, "Tree / Palette (Ctrl+B)")

ttk.Separator(toolbar, orient=tk.VERTICAL).pack(side=tk.LEFT, fill=tk.Y, padx=8, pady=2)

btn_back = ttk.Button(toolbar, text="◀", width=3, command=go_back, state="disabled")
btn_back.pack(side=tk.LEFT)
_ToolTip(btn_back, "Back (Alt+Left)")

btn_forward = ttk.Button(toolbar, text="▶", width=3, command=go_forward, state="disabled")
btn_forward.pack(side=tk.LEFT, padx=(3, 0))
_ToolTip(btn_forward, "Forward (Alt+Right)")

ttk.Separator(toolbar, orient=tk.VERTICAL).pack(side=tk.LEFT, fill=tk.Y, padx=8, pady=2)

_btn_zoom_out = ttk.Button(toolbar, text="−", width=3, command=do_zoom_out)
_btn_zoom_out.pack(side=tk.LEFT)
_ToolTip(_btn_zoom_out, "Zoom Out (Ctrl+−)")

zoom_label = ttk.Label(toolbar, text="100%", width=6, anchor="center")
zoom_label.pack(side=tk.LEFT, padx=1)

_btn_zoom_in = ttk.Button(toolbar, text="+", width=3, command=do_zoom_in)
_btn_zoom_in.pack(side=tk.LEFT)
_ToolTip(_btn_zoom_in, "Zoom In (Ctrl++)")

_btn_zoom_fit = ttk.Button(toolbar, text="⛶", width=3, command=do_zoom_fit)
_btn_zoom_fit.pack(side=tk.LEFT, padx=(3, 0))
_ToolTip(_btn_zoom_fit, "Zoom to Fit (Ctrl+9)")

_btn_tidy = ttk.Button(toolbar, text="⊞", width=3, command=do_tidy_layout)
_btn_tidy.pack(side=tk.LEFT, padx=(3, 0))
_ToolTip(_btn_tidy, "Tidy Layout (Ctrl+W)")

ttk.Separator(toolbar, orient=tk.VERTICAL).pack(side=tk.LEFT, fill=tk.Y, padx=8, pady=2)

_btn_print = ttk.Button(toolbar, text="🖨", width=3, command=wiresheet.print_sheet)
_btn_print.pack(side=tk.LEFT)
_ToolTip(_btn_print, "Print Wiresheet (Ctrl+P)")

paned_window = ttk.PanedWindow(window, orient=tk.HORIZONTAL)
paned_window.pack(fill=tk.BOTH, expand=True, padx=5, pady=5)

# Left column splits into the navigation tree above and the component palette below.
left_pane = ttk.PanedWindow(paned_window, orient=tk.VERTICAL)

frame_tree = ttk.Frame(left_pane)

_nav_style = ttk.Style()
_nav_style.map(
    app_state.NAV_TREE_STYLE,
    # No ("selected", ...) entry: without one the row keeps whatever foreground its tag
    # gives it, which is the whole point.
    foreground=[("disabled", "SystemGrayText")],
    background=[("disabled", "SystemButtonFace"),
                ("selected", app_state.NAV_SELECTION_BG)],
)

tree = ttk.Treeview(
    frame_tree,
    show="tree",
    selectmode="browse",
    height=TREE_REQUESTED_ROWS,
    style=app_state.NAV_TREE_STYLE,
)
# Configured once, for the life of the tree. Only the row's tags change after this.
tree.tag_configure(app_state.DIRTY_NAME_TAG, foreground=app_state.DIRTY_NAME_COLOUR)
app_state.tree = tree
attach_tree_scrollbars(frame_tree, tree)
tree.bind("<<TreeviewSelect>>", on_tree_select)

tree.bind("<ButtonPress-3>", on_tree_right_click)
tree.bind("<ButtonPress-2>", on_tree_right_click)

left_pane.add(frame_tree, weight=3)

# --- Component palette ---
frame_palette = ttk.Frame(left_pane)

ttk.Label(frame_palette, text="Palette", font=("TkDefaultFont", 9, "bold")).pack(
    anchor="w", padx=4, pady=(4, 2)
)

palette_kit_var = tk.StringVar()
palette_kit_combo = ttk.Combobox(frame_palette, textvariable=palette_kit_var, state="readonly")
palette_kit_combo.pack(fill=tk.X, padx=4)

frame_palette_list = ttk.Frame(frame_palette)
frame_palette_list.pack(fill=tk.BOTH, expand=True, padx=4, pady=4)

palette_tree = ttk.Treeview(
    frame_palette_list,
    show="tree",
    selectmode="browse",
    height=PALETTE_REQUESTED_ROWS,
)
attach_tree_scrollbars(frame_palette_list, palette_tree)

palette_status = ttk.Label(frame_palette, text="", foreground="#666666")
palette_status.pack(anchor="w", padx=4, pady=(0, 4))


def populate_palette_types(event=None):
    """Lists the component types of the kit chosen in the palette dropdown."""
    for item in palette_tree.get_children():
        palette_tree.delete(item)

    index = palette_kit_combo.current()
    if index < 0 or index >= len(palette_kit_names):
        palette_status.config(text="")
        return

    kit_name = palette_kit_names[index]
    types = palette.list_component_types(kit_name)

    for type_name, base in types:
        palette_tree.insert("", "end", text=type_name, values=(f"{kit_name}::{type_name}", base))

    palette_status.config(text=f"{len(types)} component{'' if len(types) == 1 else 's'}")
    fit_tree_column(palette_tree)


def refresh_palette_kits():
    """Reloads the kit list, showing only kits declared in the app's schema."""
    global palette_kit_names

    palette.invalidate()

    if _schema_has_incomplete_checksums():
        palette_kit_names = []
    else:
        palette_kit_names = sorted(app_state.active_schema_kits.keys())

    labels = []
    for kit_name in palette_kit_names:
        version = palette.kit_version_label(kit_name)
        labels.append(f"{kit_name} ({version})" if version else kit_name)

    palette_kit_combo.config(values=labels)

    for item in palette_tree.get_children():
        palette_tree.delete(item)

    if not palette_kit_names:
        if app_state.xml_root_element is None:
            palette_status.config(text="")
        elif not app_state.SEDONA_HOME:
            palette_status.config(text="Set Sedona home in Preferences")
        else:
            palette_status.config(text="No kits found — check Sedona home")
        palette_kit_var.set("")
        return

    palette_status.config(text=f"{len(palette_kit_names)} kits")
    palette_kit_var.set("")
    fit_tree_column(palette_tree)



# --- Palette drag and drop ---
# Tk has no drag-and-drop between widgets, so the gesture is assembled by hand:
# remember what was pressed, start dragging past a small threshold, follow the pointer
# with a borderless ghost window, and on release test the pointer against the canvas.
DRAG_THRESHOLD = 6

palette_drag = {"kit": None, "type": None, "origin": None, "active": False, "ghost": None}


def _destroy_drag_ghost():
    if palette_drag["ghost"] is not None:
        palette_drag["ghost"].destroy()
        palette_drag["ghost"] = None


def _reset_palette_drag():
    _destroy_drag_ghost()
    palette_drag.update(kit=None, type=None, origin=None, active=False)


def on_palette_press(event):
    item = palette_tree.identify_row(event.y)
    if not item:
        _reset_palette_drag()
        return

    palette_tree.selection_set(item)
    index = palette_kit_combo.current()
    if index < 0 or index >= len(palette_kit_names):
        return

    palette_drag.update(
        kit=palette_kit_names[index],
        type=palette_tree.item(item, "text"),
        origin=(event.x_root, event.y_root),
        active=False,
    )


def on_palette_motion(event):
    if palette_drag["type"] is None or palette_drag["origin"] is None:
        return

    origin_x, origin_y = palette_drag["origin"]
    moved = abs(event.x_root - origin_x) + abs(event.y_root - origin_y)

    if not palette_drag["active"]:
        if moved < DRAG_THRESHOLD:
            return
        palette_drag["active"] = True

        ghost = tk.Toplevel(window)
        ghost.overrideredirect(True)
        ghost.attributes("-topmost", True)
        tk.Label(
            ghost, text=f"{palette_drag['type']}",
            bg="#dce6f2", fg="black", relief="solid", borderwidth=1,
            padx=6, pady=2, font=("TkDefaultFont", 9),
        ).pack()
        palette_drag["ghost"] = ghost

    if palette_drag["ghost"] is not None:
        palette_drag["ghost"].geometry(f"+{event.x_root + 12}+{event.y_root + 10}")


def _pointer_over_canvas(x_root, y_root):
    left = canvas.winfo_rootx()
    top = canvas.winfo_rooty()
    return (left <= x_root < left + canvas.winfo_width()
            and top <= y_root < top + canvas.winfo_height())


def on_palette_release(event):
    was_dragging = palette_drag["active"]
    kit_name = palette_drag["kit"]
    type_name = palette_drag["type"]
    _reset_palette_drag()

    if not was_dragging or not kit_name or not type_name:
        return

    if not _pointer_over_canvas(event.x_root, event.y_root):
        return

    if app_state.xml_root_element is None:
        messagebox.showwarning("Add Component", "Open a SAX file first.")
        return

    # Pointer position -> canvas coordinates -> grid units, snapped like a dragged box.
    drop_x = canvas.canvasx(event.x_root - canvas.winfo_rootx())
    drop_y = canvas.canvasy(event.y_root - canvas.winfo_rooty())

    step = app_state.meta_scale * app_state.zoom
    min_step = wiresheet.layout_margin(app_state.meta_scale) // app_state.meta_scale
    grid_x = max(min_step, min(wiresheet.META_COORD_MAX, round(drop_x / step)))
    grid_y = max(min_step, min(wiresheet.META_COORD_MAX, round(drop_y / step)))

    sax_file.add_component(kit_name, type_name, grid_x, grid_y)


palette_kit_names = []
palette_kit_combo.bind("<<ComboboxSelected>>", populate_palette_types)
palette_tree.bind("<ButtonPress-1>", on_palette_press)
palette_tree.bind("<B1-Motion>", on_palette_motion)
palette_tree.bind("<ButtonRelease-1>", on_palette_release)

left_pane.add(frame_palette, weight=2)

# weight=0 keeps the tree/palette column out of the share-out of spare width, so growing
# the window feeds the wiresheet alone and the column stays at its configured width.
paned_window.add(left_pane, weight=0)

frame_editor = ttk.Frame(paned_window)

canvas_vscroll = ttk.Scrollbar(frame_editor, orient=tk.VERTICAL)
canvas_vscroll.pack(side=tk.RIGHT, fill=tk.Y)

canvas_hscroll = ttk.Scrollbar(frame_editor, orient=tk.HORIZONTAL)
canvas_hscroll.pack(side=tk.BOTTOM, fill=tk.X)

canvas = tk.Canvas(
    frame_editor,
    bg="white",
    highlightthickness=0,
    xscrollcommand=canvas_hscroll.set,
    yscrollcommand=canvas_vscroll.set,
    scrollregion=(0, 0, 2000, 2000),
)
app_state.canvas = canvas
canvas.pack(fill=tk.BOTH, expand=True)

canvas_vscroll.config(command=canvas.yview)
canvas_hscroll.config(command=canvas.xview)

canvas.bind("<ButtonPress-1>", on_canvas_click)
canvas.bind("<B1-Motion>", on_canvas_drag)
canvas.bind("<ButtonRelease-1>", on_canvas_release)
canvas.bind("<Configure>", lambda e: wiresheet.draw_grid())

canvas.bind("<ButtonPress-3>", on_canvas_right_click)
canvas.bind("<ButtonPress-2>", on_canvas_right_click)

# Wheel zoom. Windows and macOS report a delta; X11 sends button 4 and 5 instead.
canvas.bind("<Control-MouseWheel>", on_canvas_wheel_zoom)
canvas.bind("<Control-Button-4>", do_zoom_in)
canvas.bind("<Control-Button-5>", do_zoom_out)

paned_window.add(frame_editor, weight=3)

app_state.dirty_display_callback = refresh_dirty_display
undo.set_refresh_callback(refresh_all_views)
sax_file.set_tree_filled_callback(on_tree_filled)
refresh_palette_kits()
apply_left_pane_width()

# Moving the sash changes the split directly; resizing the window changes what fits.
left_pane.bind("<B1-Motion>", clamp_left_pane_split, add="+")
left_pane.bind("<ButtonRelease-1>", clamp_left_pane_split, add="+")
left_pane.bind("<Configure>", clamp_left_pane_split, add="+")

window.after(100, verify_startup_configurations)
window.mainloop()