Wiresheet: arrow nudges keep the selection and the sheet's focus

Ctrl+A hands the sheet the keyboard, and repainting the same sheet keeps
whatever of the selection is still on it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

app_state.py

@@ -8,7 +8,7 @@ so later rebinding would only ever be seen by the module that did it.
 import configparser
 import os
 
-VERSION = "0.0.0.173"
+VERSION = "0.0.0.180"
 PROPERTIES_FILE = os.path.join(os.path.dirname(os.path.abspath(__file__)), "editor.properties")
 DEFAULT_GRID_SIZE = 10
 DEFAULT_ADD_TIMESTAMP = True
@@ -41,6 +41,9 @@ status_label = None      # the window's bottom status bar
 # colour wherever the change was made.
 DIRTY_TINT = "#ffe6e6"
 CLEAN_TINT = "white"
+# The wiresheet canvas around the sheet itself, clean and with unsaved changes.
+OFF_SHEET_TINT = "#f3f3f3"
+OFF_SHEET_DIRTY_TINT = "#f0dcdc"
 
 # The file name on the tree's root row turns this colour while anything is unwritten,
 # which is the one place a file name appears on screen -- a Tk window title cannot be
@@ -239,8 +242,7 @@ def mark_dirty():
     global is_dirty
     if not is_dirty:
         is_dirty = True
-        if canvas is not None:
-            canvas.config(bg=DIRTY_TINT)
+        paint_sheet_tint()
     refresh_dirty_display()
 
 
@@ -248,11 +250,26 @@ def mark_clean():
     """Clears the unsaved-state marker after a successful save or a fresh load."""
     global is_dirty
     is_dirty = False
-    if canvas is not None:
-        canvas.config(bg=CLEAN_TINT)
+    paint_sheet_tint()
     refresh_dirty_display()
 
 
+def sheet_tints():
+    """(sheet, off-sheet) colours for the wiresheet in the current saved/unsaved state."""
+    if is_dirty:
+        return DIRTY_TINT, OFF_SHEET_DIRTY_TINT
+    return CLEAN_TINT, OFF_SHEET_TINT
+
+
+def paint_sheet_tint():
+    """Recolours the sheet (the "sheet_bg" rectangle) and the canvas around it."""
+    if canvas is None:
+        return
+    sheet, off_sheet = sheet_tints()
+    canvas.config(bg=off_sheet)
+    canvas.itemconfig("sheet_bg", fill=sheet)
+
+
 # ── Live device connection state ────────────────────────────────
 live_mode = False
 device_session = None       # device_session.DeviceSession instance

link_drag.py

@@ -11,7 +11,9 @@ either end. While dragging, the cursor points along the flow of the link over a
 would take it, and turns into a no-entry sign over one that would not, so a bad drop is
 visible before the button is released.
 
-A right press that never moves still opens the component's context menu. A left press
+A right press that never moves still opens the component's context menu. A left drag
+that starts on bare sheet draws a selection rectangle, and selects the boxes wholly
+inside it on release; with Ctrl held they join the current selection. A left press
 anywhere on a box selects it, but only a press on its title bar moves it; on a slot row
 it draws a link instead, as in Workbench.
 """
@@ -33,11 +35,15 @@ HOVER_COLOR = "#1f4e79"
 DROP_OK_COLOR = "#2e9d3a"
 DROP_BAD_COLOR = "#d62828"
 RUBBER_COLOR = "#2f6fba"
+MARQUEE_COLOR = "#2f6fba"
+CTRL_MASK = 0x0004
 
 _cursors = {}                   # role -> Tk cursor spec, built on first use
 _press = None                   # pending right press: {"hit", "x", "y", "event"}
 _drag = None                    # live link drag: {"hit", "anchor"}
+_marquee = None                 # left press on bare sheet: {"x", "y", "cx", "cy", "state", "active"}
 _menu_callback = None           # opens the ordinary right-click menu for an event
+_select_boxes = None            # makes a list of comp tags the box selection
 _link_dialog = None             # opens the Link dialog: (source, target, src_slot, tgt_slot)
 
 
@@ -301,14 +307,16 @@ def _anchor(hit):
 
 def clear():
     """Drops any hover ring, rubber band and drag state; called on every repaint."""
-    global _press, _drag
+    global _press, _drag, _marquee
     canvas = app_state.canvas
     if canvas is None:
         return
     canvas.delete("slot_hover")
     canvas.delete("link_rubber")
+    canvas.delete("marquee")
     _press = None
     _drag = None
+    _marquee = None
     _set_cursor(None)
 
 
@@ -442,6 +450,67 @@ def on_right_release(event):
         _menu_callback(event)
 
 
+# Left button on bare sheet: dragging pulls out a selection rectangle that selects every
+# box wholly inside it on release (Ctrl adds them to the current selection).
+
+def _on_bare_sheet(cx, cy):
+    """True if no box or link lies under a canvas point."""
+    canvas = app_state.canvas
+    for item in canvas.find_overlapping(cx - 3, cy - 3, cx + 3, cy + 3):
+        tags = canvas.gettags(item)
+        if "component" in tags or "link" in tags:
+            return False
+    return True
+
+
+def _marquee_motion(event):
+    """Draws the selection rectangle once the pointer has moved far enough."""
+    global _marquee
+    if _marquee is None:
+        return
+    if not _marquee["active"]:
+        if abs(event.x - _marquee["x"]) < DRAG_THRESHOLD and \
+                abs(event.y - _marquee["y"]) < DRAG_THRESHOLD:
+            return
+        _marquee["active"] = True
+        app_state.canvas.delete("slot_hover")
+    canvas = app_state.canvas
+    canvas.delete("marquee")
+    canvas.create_rectangle(_marquee["cx"], _marquee["cy"],
+                            canvas.canvasx(event.x), canvas.canvasy(event.y),
+                            outline=MARQUEE_COLOR, width=1, dash=(4, 2),
+                            tags=("marquee",))
+    canvas.tag_raise("marquee")
+
+
+def _marquee_finish(event):
+    """Ends a rectangle drag. Returns True if it selected, False if it was a click."""
+    global _marquee
+    marquee, _marquee = _marquee, None
+    canvas = app_state.canvas
+    canvas.delete("marquee")
+    if marquee is None or not marquee["active"]:
+        return False
+    cx, cy = canvas.canvasx(event.x), canvas.canvasy(event.y)
+    left, right = sorted((marquee["cx"], cx))
+    top, bottom = sorted((marquee["cy"], cy))
+    inside = []
+    for comp_tag in app_state.canvas_comp_map:
+        box = wiresheet.get_box_bounds(comp_tag)
+        if box and left <= box[0] and top <= box[1] and box[2] <= right and box[3] <= bottom:
+            inside.append(comp_tag)
+    adding = (marquee["state"] | event.state) & CTRL_MASK
+    wiresheet.deselect_link()        # links and boxes are never selected together
+    if adding:
+        current = [t for t in app_state.selected_canvas_tags if t not in inside]
+        _select_boxes(current + inside)
+    else:
+        _select_boxes(inside)
+    count = len(app_state.selected_canvas_tags)
+    app_state.set_status(f"{count} component{'s' if count != 1 else ''} selected.")
+    return True
+
+
 # Left button: anywhere on a box selects it. Only the title bar moves it; a slot row
 # drags out a link, and the rest of the box does nothing more than select.
 
@@ -451,6 +520,19 @@ def _on_title_bar(comp_tag, cy):
 
 
 def on_left_press(event):
+    global _marquee
+    _marquee = None
+    canvas = app_state.canvas
+    cx, cy = canvas.canvasx(event.x), canvas.canvasy(event.y)
+    if _on_bare_sheet(cx, cy):
+        app_state.drag_data["tag"] = None
+        if not event.state & CTRL_MASK:
+            # A plain click on bare sheet still clears the selection, drag or not.
+            wiresheet.deselect_link()
+            _select_boxes([])
+        _marquee = {"x": event.x, "y": event.y, "cx": cx, "cy": cy,
+                    "state": event.state, "active": False}
+        return
     _box_handlers["press"](event)               # selection, and arms a box move
     if _start_press(event) is not None:
         app_state.canvas.delete("slot_hover")
@@ -460,30 +542,37 @@ def on_left_press(event):
 
 
 def on_left_motion(event):
-    if _press is not None or _drag is not None:
+    if _marquee is not None:
+        _marquee_motion(event)
+    elif _press is not None or _drag is not None:
         _drag_motion(event)
     else:
         _box_handlers["motion"](event)
 
 
 def on_left_release(event):
-    if _finish(event) is None:
+    if _marquee is not None:
+        _marquee_finish(event)
+    elif _finish(event) in (None, "click"):
         _box_handlers["release"](event)
 
 
 _box_handlers = {}
 
 
-def install(canvas, menu_callback, box_press, box_motion, box_release, link_dialog):
+def install(canvas, menu_callback, box_press, box_motion, box_release, link_dialog,
+            select_boxes):
     """Binds hover, link dragging on both buttons, and the box handlers it wraps.
 
     menu_callback opens the context menu; box_* are the sheet's own left-button
     select-and-move handlers, which still run whenever a press is not on a slot row;
-    link_dialog opens the Link dialog when a link is dropped on a title bar.
+    link_dialog opens the Link dialog when a link is dropped on a title bar;
+    select_boxes makes a list of comp tags the selection after a rectangle drag.
     """
-    global _menu_callback, _link_dialog
+    global _menu_callback, _link_dialog, _select_boxes
     _menu_callback = menu_callback
     _link_dialog = link_dialog
+    _select_boxes = select_boxes
     _box_handlers.update(press=box_press, motion=box_motion, release=box_release)
     canvas.bind("<Motion>", on_motion)
     canvas.bind("<Leave>", on_leave, add="+")

main.py

@@ -933,7 +933,7 @@ def do_tidy_layout(event=None):
     multiplier = app_state.meta_scale // app_state.GRID_SIZE
 
     if moved == 0:
-        messagebox.showinfo("Tidy Layout", "Nothing to move — no boxes overlap.")
+        messagebox.showinfo("Tidy Layout", "Nothing to move — the sheet is already tidy.")
     elif clamped:
         messagebox.showwarning(
             "Tidy Layout",
@@ -991,14 +991,8 @@ def zoom_to(level, anchor=None):
     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))
+    # Scroll the repainted sheet back under the held point.
+    wiresheet.scroll_view_to(sheet_x * after - anchor[0], sheet_y * after - anchor[1])
 
 
 def do_zoom_in(event=None):
@@ -1054,12 +1048,8 @@ def do_zoom_fit(event=None):
     applied = wiresheet.set_zoom(level)
     update_zoom_label()
 
-    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))
+    wiresheet.scroll_view_to(bounds[0] * applied - wiresheet.FIT_PADDING,
+                             bounds[1] * applied - wiresheet.FIT_PADDING)
     return "break"
 
 
@@ -1256,16 +1246,11 @@ 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:
+    if box is None:
         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))
+    wiresheet.scroll_view_to((box[0] + box[2]) / 2 - canvas.winfo_width() / 2,
+                             (box[1] + box[3]) / 2 - canvas.winfo_height() / 2)
 
 
 def select_link_element(link_elem):
@@ -1359,6 +1344,26 @@ def select_link_on_open_sheet(link_elem):
         app_state.set_status("That link is not painted on this sheet.", warn=True)
 
 
+def select_all_boxes(event=None):
+    """Ctrl+A: selects every box on the open wiresheet."""
+    # Bound application-wide like Del, so a text field keeps its own select-all.
+    if editor_panel_showing():
+        return None
+    focused = app_state.window.focus_get()
+    if focused is not None:
+        if focused.winfo_toplevel() is not app_state.window:
+            return None
+        if focused.winfo_class() in TEXT_ENTRY_CLASSES:
+            return None
+    wiresheet.deselect_link()        # links and boxes are never selected together
+    select_canvas_boxes(list(app_state.canvas_comp_map))
+    # Hand the sheet the keyboard, so the arrows move the boxes rather than the tree.
+    app_state.canvas.focus_set()
+    count = len(app_state.selected_canvas_tags)
+    app_state.set_status(f"{count} component{'s' if count != 1 else ''} selected.")
+    return "break"
+
+
 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
@@ -1547,6 +1552,15 @@ def on_canvas_click(event):
         _toggle_canvas_box(group_tag)
         return
 
+    # Pressing one of several selected boxes keeps them all selected, so a drag moves
+    # them together; a press that never moves narrows the selection to it on release.
+    keep_group = (group_tag and not wire_tag and group_tag in app_state.selected_canvas_tags
+                  and len(app_state.selected_canvas_tags) > 1)
+    app_state.drag_data["group"] = []
+    app_state.drag_data["moved"] = False
+    app_state.drag_data["collapse_to"] = group_tag if keep_group else None
+
+    if not keep_group:
         select_canvas_boxes([])
     if wire_tag:
         wiresheet.select_link(wire_tag)
@@ -1556,20 +1570,26 @@ def on_canvas_click(event):
         app_state.drag_data["tag"] = group_tag
         app_state.drag_data["x"] = cx
         app_state.drag_data["y"] = cy
+        if keep_group:
+            # Del and the menus still act on the box pressed last.
+            app_state.selected_canvas_tag = group_tag
+        else:
             select_canvas_boxes([group_tag])
 
-        # Where the box starts out. The drag is measured as an offset from this pair
+        # Where each moving box starts out. The drag is measured as an offset from these
         # 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
-
-        # Bring the clicked box clear of any neighbour it happens to sit under.
+        # Each entry also snapshots the pre-drag meta, so the move is one undoable step.
+        moving = list(app_state.selected_canvas_tags) if keep_group else [group_tag]
+        for tag in moving:
+            origin = wiresheet.get_box_origin(tag)
+            comp = app_state.canvas_comp_map.get(tag)
+            meta_prop = comp.find("prop[@name='meta']") if comp is not None else None
+            app_state.drag_data["group"].append({
+                "tag": tag, "origin": origin,
+                "meta_before": meta_prop.get("val") if meta_prop is not None else None,
+            })
+            # Bring the moving boxes clear of any neighbour they happen to sit under.
+            canvas.tag_raise(tag)
         canvas.tag_raise(group_tag)
         wiresheet.redraw_links()
 
@@ -2086,18 +2106,17 @@ def on_canvas_drag(event):
         return
 
     canvas = app_state.canvas
-    group_tag = app_state.drag_data["tag"]
     cx = canvas.canvasx(event.x)
     cy = canvas.canvasy(event.y)
+    group = app_state.drag_data.get("group") or []
 
-    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"])
+    if not group or any(entry["origin"] is None for entry in group):
+        # Nothing to align against, so the boxes just follow the pointer one to one.
+        for tag in [entry["tag"] for entry in group] or [app_state.drag_data["tag"]]:
+            canvas.move(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
+        app_state.drag_data["moved"] = True
         wiresheet.redraw_links()
         return
 
@@ -2106,39 +2125,68 @@ def on_canvas_drag(event):
     # 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
+    # A box's corner also stays on the sheet, the furthest meta can store it.
+    limit = wiresheet.META_COORD_MAX * step
 
-    # 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)
+    # Follow the pointer, but keep every moving box out of the margin strip along the
+    # top and left canvas edges and on the sheet -- the whole group stops together, so
+    # the boxes keep their places relative to one another.
+    dx = cx - app_state.drag_data["x"]
+    dy = cy - app_state.drag_data["y"]
+    xs = [entry["origin"][0] for entry in group]
+    ys = [entry["origin"][1] for entry in group]
+    dx = min(limit - max(xs), max(margin - min(xs), dx))
+    dy = min(limit - max(ys), max(margin - min(ys), dy))
 
+    shifted = False
+    for entry in group:
+        # Land each box on a step every step of the way.
+        start_x, start_y = entry["origin"]
+        target_x = min(limit, max(margin, round((start_x + dx) / step) * step))
+        target_y = min(limit, max(margin, round((start_y + dy) / step) * step))
+        origin = wiresheet.get_box_origin(entry["tag"])
+        if origin is None:
+            continue
         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
+            canvas.move(entry["tag"], shift_x, shift_y)
+            shifted = True
+
+    if shifted:
+        app_state.drag_data["moved"] = True
+        # Wires are not part of the dragged groups, so they are re-routed against the
         # new positions.
         wiresheet.redraw_links()
 
 
 def on_canvas_release(event):
+    collapse_to = app_state.drag_data.pop("collapse_to", None)
+    if collapse_to and not app_state.drag_data.get("moved"):
+        # A click that never moved on one of several selected boxes picks it alone.
+        select_canvas_boxes([collapse_to])
+
     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)
+    margin = wiresheet.layout_margin(app_state.meta_scale) * app_state.zoom
+    limit = wiresheet.META_COORD_MAX * step
+    group = app_state.drag_data.get("group") or [
+        {"tag": app_state.drag_data["tag"], "meta_before": None}]
 
-    if origin is not None:
+    changes = []
+    failed = []
+    for entry in group:
+        group_tag = entry["tag"]
+        origin = wiresheet.get_box_origin(group_tag)
+        if origin is None:
+            continue
         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)
+        snapped_x = min(limit, max(margin, round(current_x / step) * step))
+        snapped_y = min(limit, max(margin, round(current_y / step) * step))
 
         shift_x = snapped_x - current_x
         shift_y = snapped_y - current_y
@@ -2151,15 +2199,17 @@ def on_canvas_release(event):
         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:
+        if comp_element is None:
+            continue
         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)
+        if orig_x == raw_x and orig_y == raw_y:
+            continue
+        meta_before = entry.get("meta_before") or meta_str
         update_meta_property(comp_element, meta_str, raw_x, raw_y)
         if app_state.live_mode:
             new_meta = comp_element.find("prop[@name='meta']")
@@ -2170,24 +2220,70 @@ def on_canvas_release(event):
                     remove_prop(comp_element, "meta")
                 else:
                     set_prop_value(comp_element, "meta", meta_str)
-                        app_state.drag_data["tag"] = None
-                        wiresheet.render_current_sheet()
-                        live_hooks.warn_failure(
-                            "Move", f"{comp_element.get('name', 'The component')} was not "
-                            "moved on the device")
-                        return
-                command_class = (undo.LiveMetaMoveCommand if app_state.live_mode
-                                 else undo.MetaMoveCommand)
-                undo.push(command_class(
-                    [(comp_element, meta_before, raw_x, raw_y)],
-                    label=f"move {comp_element.get('name', 'component')}"
-                ))
-                app_state.mark_dirty()
+                failed.append(comp_element.get("name", "?"))
+                continue
+        changes.append((comp_element, meta_before, raw_x, raw_y))
 
     app_state.drag_data["tag"] = None
+    app_state.drag_data["group"] = []
+
+    if changes:
+        command_class = (undo.LiveMetaMoveCommand if app_state.live_mode
+                         else undo.MetaMoveCommand)
+        label = (f"move {changes[0][0].get('name', 'component')}" if len(changes) == 1
+                 else f"move {len(changes)} components")
+        undo.push(command_class(changes, label=label))
+        app_state.mark_dirty()
+
+    if failed:
+        wiresheet.render_current_sheet()
+        live_hooks.warn_failure("Move", f"{', '.join(failed)} not moved on the device")
+        return
+
     wiresheet.redraw_links()
 
 
+def nudge_selected_boxes(dx, dy):
+    """Arrow keys: moves every selected box one meta step, as one undoable move."""
+    canvas = app_state.canvas
+    tags = [t for t in app_state.selected_canvas_tags if t in app_state.canvas_comp_map]
+    if not tags or app_state.drag_data.get("tag"):
+        return "break"
+    step = app_state.meta_scale * app_state.zoom
+    margin = wiresheet.layout_margin(app_state.meta_scale) * app_state.zoom
+    limit = wiresheet.META_COORD_MAX * step
+    origins = {t: wiresheet.get_box_origin(t) for t in tags}
+    if any(o is None for o in origins.values()):
+        return "break"
+    # The group stops as one at the margin strip and the sheet's far edge.
+    shift_x = min(limit - max(o[0] for o in origins.values()),
+                  max(margin - min(o[0] for o in origins.values()), dx * step))
+    shift_y = min(limit - max(o[1] for o in origins.values()),
+                  max(margin - min(o[1] for o in origins.values()), dy * step))
+    if not shift_x and not shift_y:
+        return "break"
+
+    # Moved on the canvas, then written back exactly as a finished mouse drag is.
+    group = []
+    for tag in tags:
+        comp = app_state.canvas_comp_map[tag]
+        meta_prop = comp.find("prop[@name='meta']")
+        group.append({"tag": tag, "origin": origins[tag],
+                      "meta_before": meta_prop.get("val") if meta_prop is not None else None})
+        canvas.move(tag, shift_x, shift_y)
+    app_state.drag_data.update(tag=tags[0], group=group, moved=True, collapse_to=None)
+    pressed_last = app_state.selected_canvas_tag
+    on_canvas_release(None)
+    # The same boxes stay selected, and the sheet keeps the keyboard, so the next
+    # arrow press moves them again.
+    still_here = [t for t in tags if t in app_state.canvas_comp_map and t != pressed_last]
+    if pressed_last in app_state.canvas_comp_map:
+        still_here.append(pressed_last)
+    select_canvas_boxes(still_here)
+    canvas.focus_set()
+    return "break"
+
+
 def fit_tree_column(tree):
     """Sizes a Treeview's tree column to its widest visible row.
 
@@ -3261,6 +3357,8 @@ SHORTCUTS = [
     ("9", do_zoom_fit),
     ("KP_9", do_zoom_fit),
     ("p", wiresheet.print_sheet),
+    ("a", select_all_boxes),
+    ("A", select_all_boxes),
 ]
 for key, handler in SHORTCUTS:
     window.bind_all(f"<Command-{key}>", handler)
@@ -3755,11 +3853,11 @@ canvas_hscroll.pack(side=tk.BOTTOM, fill=tk.X)
 
 canvas = tk.Canvas(
     frame_editor,
-    bg="white",
+    bg=app_state.OFF_SHEET_TINT,
     highlightthickness=0,
     xscrollcommand=canvas_hscroll.set,
     yscrollcommand=canvas_vscroll.set,
-    scrollregion=(0, 0, 2000, 2000),
+    scrollregion=(0, 0, *wiresheet.sheet_extent()),
 )
 app_state.canvas = canvas
 canvas.pack(fill=tk.BOTH, expand=True)
@@ -3773,8 +3871,13 @@ canvas.bind("<Configure>", lambda e: wiresheet.draw_grid())
 # selects and moves boxes and a plain right click is the menu, as before.
 link_drag.install(canvas, on_canvas_right_click,
                   on_canvas_click, on_canvas_drag, on_canvas_release,
-                  link_dialog=open_link_dialog)
+                  link_dialog=open_link_dialog, select_boxes=select_canvas_boxes)
 canvas.bind("<ButtonPress-2>", on_canvas_right_click)
+# A click gives the sheet the keyboard, so the arrows nudge its selected boxes rather
+# than walking whatever tree or list last had focus.
+canvas.bind("<ButtonPress-1>", lambda e: canvas.focus_set(), add="+")
+for _key, _dx, _dy in (("Left", -1, 0), ("Right", 1, 0), ("Up", 0, -1), ("Down", 0, 1)):
+    canvas.bind(f"<{_key}>", lambda e, dx=_dx, dy=_dy: nudge_selected_boxes(dx, dy))
 canvas.bind("<Double-1>", on_canvas_double_click)
 
 # Wheel zoom. Windows and macOS report a delta; X11 sends button 4 and 5 instead.

sax_file.py

@@ -180,7 +180,7 @@ def reset_to_startup_state():
 
     if app_state.canvas is not None:
         app_state.canvas.delete("all")
-        wiresheet.draw_grid()
+        wiresheet.fit_sheet_region()
 
     if app_state.window is not None:
         app_state.window.title(app_state.base_window_title())
@@ -223,7 +223,7 @@ def open_file():
         for item in tree.get_children():
             tree.delete(item)
         canvas.delete("all")
-        wiresheet.draw_grid()
+        wiresheet.fit_sheet_region()
 
         xml_tree = ET.parse(file_path)
         app_state.xml_root_element = xml_tree.getroot()

sox.py

@@ -134,7 +134,9 @@ class SoxMsg:
 
     def write_i4(self, v):
         self._grow(4)
-        struct.pack_into(">i", self._buf, self._pos, v)
+        # Wrapped to signed: meta packs x into the top byte, so x >= 128 reads as an
+        # unsigned 32-bit value although the device stores the same bits as sys::int.
+        struct.pack_into(">i", self._buf, self._pos, ((v + 2**31) % 2**32) - 2**31)
         self._pos += 4
 
     def write_i8(self, v):

wiresheet.py

@@ -122,6 +122,7 @@ FIT_PADDING = 20
 # Below this a grid line is closer to its neighbour than the line is wide, which reads
 # as a grey wash rather than a grid, so the grid is left off instead.
 MIN_GRID_STEP = 3
+SHEET_MARGIN_GRIDS = 3  # grid squares of grey kept past anything that overflows the sheet
 
 
 def snap_up(value, grid_size):
@@ -238,28 +239,42 @@ def measure_box(comp, fonts, linked_slot_index):
 
 
 def draw_grid():
-    """Draws a grid onto the Canvas based on the scrollable region.
+    """Paints the sheet and grids it; the canvas around it is the off-sheet tint.
 
-    Drawn at the sheet's zoom, so a box still lands on a line at every scale. The area
-    covered is the same patch of sheet whatever the zoom, which is why the span is
-    scaled along with the step.
+    Drawn at the sheet's zoom, so a box still lands on a line at every scale.
     """
     canvas = app_state.canvas
     canvas.delete("grid")
 
+    # The sheet starts past the margin strip no box may enter, which stays grey too.
+    x1 = y1 = layout_margin(app_state.meta_scale) * app_state.zoom
+    x2, y2 = (coord * app_state.zoom for coord in sheet_extent())
+    canvas.create_rectangle(x1, y1, x2, y2, outline="#c8c8c8", tags=("grid", "sheet_bg"))
+    app_state.paint_sheet_tint()   # white or pink by saved state, grey or pink around it
+
     step = app_state.GRID_SIZE * app_state.zoom
-    if step < MIN_GRID_STEP:
-        return
-
-    span = 3000 * app_state.zoom
-    for index in range(int(span // step) + 1):
-        offset = index * step
-        canvas.create_line(offset, 0, offset, span, fill="#e0e0e0", tags="grid")
-        canvas.create_line(0, offset, span, offset, fill="#e0e0e0", tags="grid")
-
+    if step >= MIN_GRID_STEP:
+        draw_grid_lines(canvas, step, x1, y1, x2, y2)
     canvas.tag_lower("grid")
 
 
+def sheet_extent():
+    """1:1 (width, height) of the sheet: the furthest position meta can store a box at.
+
+    Only a box's top-left corner has to fit; a box reaching past the edge is drawn over
+    the grey, and keeping it inside is left to the author.
+    """
+    side = META_COORD_MAX * (app_state.meta_scale or app_state.GRID_SIZE)
+    return side, side
+
+
+def draw_grid_lines(canvas, step, x1, y1, x2, y2):
+    for index in range(math.ceil(x1 / step - 1e-6), math.floor(x2 / step + 1e-6) + 1):
+        canvas.create_line(index * step, y1, index * step, y2, fill="#e0e0e0", tags="grid")
+    for index in range(math.ceil(y1 / step - 1e-6), math.floor(y2 / step + 1e-6) + 1):
+        canvas.create_line(x1, index * step, x2, index * step, fill="#e0e0e0", tags="grid")
+
+
 def create_round_rectangle(canvas, x1, y1, x2, y2, radius=4, **kwargs):
     """Draws a visually smooth rounded rectangle using native canvas arcs and polygons."""
     points = [
@@ -337,18 +352,32 @@ def segment_hits_boxes(x1, y1, x2, y2, boxes):
 def path_is_clear(points, boxes, clear):
     """True if no part of the path runs under a box or crowds one.
 
-    Every run between the first turn and the last keeps `clear` from each box: `end`
-    above and below, `side` left and right. The two runs pinned to the slots are only
-    held out from under the boxes, since they have to reach their pins wherever the
-    rows happen to be.
+    Every run keeps `clear` from each box: `end` above and below, `side` left and
+    right. Only the two stubs pinned to the slots -- `side` out of the source and
+    `arrive` into the target -- are held merely out from under the boxes, since they
+    have to reach their pins wherever the rows happen to be. A run that carries on
+    level past its stub is still a run, so it may not graze a box edge on the way.
     """
     points = wire_style.simplify(points)
     padded = [(x1 - clear.side, y1 - clear.end, x2 + clear.side, y2 + clear.end)
               for x1, y1, x2, y2 in boxes]
     last = len(points) - 2
     for idx, ((x1, y1), (x2, y2)) in enumerate(zip(points, points[1:])):
-        pinned = idx in (0, last)
-        if segment_hits_boxes(x1, y1, x2, y2, boxes if pinned else padded):
+        level = abs(y1 - y2) < LINK_EPS
+        lo, hi = x1, x2             # the part of the run that must keep its distance
+        if level and idx == 0:
+            lo = x1 + math.copysign(min(clear.side, abs(x2 - x1)), x2 - x1)
+        if level and idx == last:
+            hi = x2 - math.copysign(min(clear.arrive, abs(x2 - lo)), x2 - lo)
+        if idx in (0, last) and level:
+            if segment_hits_boxes(x1, y1, x2, y2, boxes):
+                return False
+            if abs(hi - lo) > LINK_EPS and segment_hits_boxes(lo, y1, hi, y2, padded):
+                return False
+        elif idx in (0, last):
+            if segment_hits_boxes(x1, y1, x2, y2, boxes):
+                return False
+        elif segment_hits_boxes(x1, y1, x2, y2, padded):
             return False
     return True
 
@@ -433,14 +462,15 @@ def lane_is_crowded(lane, run_lo, run_hi, claimed, pins, apart):
 
 
 def free_lane(cx, run_lo, run_hi, boxes, taken, low, high, clear, max_steps=8,
-              pins=None):
+              pins=None, accept=None):
     """Nudges cx outwards in `clear.lane` steps until the run has a lane of its own.
 
     A lane is rejected while it crowds a box (closer than `clear.side`) or while it
     would run alongside an already placed run that overlaps it vertically -- two runs
     passing through the same gap have to stand `clear.lane` apart. Returns None when
     no lane inside the gap works. `max_steps` of 1 permits no nudging at all, which is
-    how the caller asks for the exact hug position and nothing else.
+    how the caller asks for the exact hug position and nothing else. `accept`, when
+    given, can turn down a lane that is otherwise free, and the search carries on.
     """
     for step in range(0, max_steps):
         for sign in ((0,) if step == 0 else (1, -1)):
@@ -449,11 +479,38 @@ def free_lane(cx, run_lo, run_hi, boxes, taken, low, high, clear, max_steps=8,
                 continue
             if vertical_run_clearance(lane, run_lo, run_hi, boxes) < clear.side - LINK_EPS:
                 continue
-            if not lane_is_crowded(lane, run_lo, run_hi, taken, pins, clear.lane):
+            if lane_is_crowded(lane, run_lo, run_hi, taken, pins, clear.lane):
+                continue
+            if accept is None or accept(lane):
                 return lane
     return None
 
 
+def crosses_wires(points, ledger, pins):
+    """True if the path would cross a run another wire has already laid down.
+
+    A crossing gets a hop, so it is allowed, but a lane that avoids one is worth
+    looking for first. Wires sharing a pin are one signal and never count.
+    """
+    points = wire_style.simplify(points)
+    for (x1, y1), (x2, y2) in zip(points, points[1:]):
+        if abs(x1 - x2) < LINK_EPS:              # vertical: against other wires' rows
+            lo, hi = min(y1, y2), max(y1, y2)
+            for ry, rx1, rx2, row_pins in ledger["rows"]:
+                if (row_pins != pins and not same_signal(pins, row_pins)
+                        and rx1 + LINK_EPS < x1 < rx2 - LINK_EPS
+                        and lo + LINK_EPS < ry < hi - LINK_EPS):
+                    return True
+        else:                                    # horizontal: against their lanes
+            lo, hi = min(x1, x2), max(x1, x2)
+            for cx, c_lo, c_hi, col_pins in ledger["cols"]:
+                if (col_pins != pins and not same_signal(pins, col_pins)
+                        and lo + LINK_EPS < cx < hi - LINK_EPS
+                        and c_lo + LINK_EPS < y1 < c_hi - LINK_EPS):
+                    return True
+    return False
+
+
 def hug_lane(edge_x, side, run_lo, run_hi, boxes, claimed, clear, max_offset=0,
              min_offset=0, pins=None):
     """Lane for a run pinned to one box edge, starting `clear.side` out from it.
@@ -550,20 +607,36 @@ def route_link_path(src_box, src_y, dst_box, dst_y, boxes, taken=None,
         return any(row_is_stacked(y, x1, x2, claimed_rows, pins, clear.row)
                    for y, x1, x2, _ in rows_of(candidate))
 
-    def straight_through(max_steps, allow_stack=False):
+    def path_through(lane):
+        return [start, leave, (lane, src_y), (lane, dst_y), arrive, end]
+
+    def straight_through(max_steps, allow_stack=False, uncrossed=False, hug_only=False):
         """A single vertical channel in the gap between the two boxes.
 
         The two horizontal runs are pinned to their slots, so unless `allow_stack`
         they are refused when they would hide under another wire: the wire is better
-        off detouring, which puts it on a lane it can actually move.
+        off detouring, which puts it on a lane it can actually move. With `uncrossed`
+        only a lane that crosses no other wire will do; with `hug_only` only the lane
+        beside its own source is tried (beside either end for a level wire), not ones
+        beside the boxes in between.
         """
+        accept = None
+        if uncrossed:
+            accept = lambda lane: not crosses_wires(path_through(lane), ledger, pins)
         candidates = channel_candidates(src_box, dst_box, boxes, low, high, clear)
+        if hug_only:
+            # A bent wire hugs its source only: turning at the far end instead would
+            # take the lane the wires below it need to turn outside it.
+            hugs = (src_box[2] + clear.hug,)
+            if abs(src_y - dst_y) < LINK_EPS:
+                hugs += (dst_box[0] - clear.arrive,)
+            candidates = [cx for cx in candidates if cx in hugs]
         for cx in candidates:
             lane = free_lane(cx, run_lo, run_hi, boxes, claimed, low, high, clear,
-                             max_steps, pins)
+                             max_steps, pins, accept)
             if lane is None:
                 continue
-            candidate = [start, leave, (lane, src_y), (lane, dst_y), arrive, end]
+            candidate = path_through(lane)
             if not path_is_clear(candidate, boxes, clear):
                 continue
             if not allow_stack and stacked(candidate):
@@ -651,10 +724,15 @@ def route_link_path(src_box, src_y, dst_box, dst_y, boxes, taken=None,
         # First pass: only the straight route, whose horizontal runs are pinned to their
         # slots and cannot be moved later. Letting these claim their rows first stops a
         # detouring wire from parking its movable lane on top of one of them.
-        attempts = (lambda: straight_through(1),)
+        # Only an uncrossed one, too: a wire that would cross another waits for the
+        # second pass, where it can look further for a lane that does not.
+        # A wire whose hug lane is taken waits as well, rather than jumping to a lane far
+        # across the gap whose long run would then shut out the wires routed after it.
+        attempts = (lambda: straight_through(1, uncrossed=True, hug_only=True),)
     else:
         standoff = max(WIRE_STANDOFF_MAX, 3 * clear.side)
         attempts = (
+            lambda: straight_through(8, uncrossed=True),  # own lane, crossing nothing
             lambda: straight_through(1),                  # hugging, own lane
             lambda: around_boxes(clear.hug),              # hugging, routed around
             lambda: straight_through(8),                  # lane nudged out of the band
@@ -876,8 +954,16 @@ def draw_links():
     jobs = collect_link_jobs(links_elem, path_to_tag, want_knob)
     boxes = [b for b in (get_box_bounds(t) for t in path_to_tag.values()) if b]
 
-    # Shorter, simpler links claim lanes first; ties go to the smaller vertical span.
-    jobs.sort(key=lambda j: (abs(j.dst_box[0] - j.src_box[2]), abs(j.dst_y - j.src_y)))
+    # Links between nearer columns claim lanes first, and straight ones before bent ones.
+    # Among bent wires leaving one column, the first routed hugs its source closest, so
+    # climbing wires go top source first and falling ones bottom source first: each
+    # later wire then turns outside the earlier ones instead of crossing them.
+    def routing_order(job):
+        bent = abs(job.dst_y - job.src_y) > LINK_EPS
+        climbing = job.dst_y < job.src_y
+        return (round(job.dst_box[0] - job.src_box[0]), bent, not climbing,
+                job.src_y if climbing else -job.src_y)
+    jobs.sort(key=routing_order)
 
     # What the wires placed so far have claimed, shared across the whole sheet.
     # Routed in two passes: a wire that can run straight has its horizontal runs pinned
@@ -887,8 +973,16 @@ def draw_links():
     # placed closer than the grid rule allows still get a wire, just a tighter one.
     roomy = grid_clearance()
     ledger = {"cols": [], "rows": [row for job in jobs for row in job.stub_rows(roomy)]}
+    # A wire that cannot go direct defers the bent wires after it across the same gap
+    # in the same direction too, so they still take their lanes in routing order.
+    deferred = set()
     for job in jobs:
+        group = (round(job.src_box[0]), round(job.dst_box[0]), job.dst_y < job.src_y)
+        if group in deferred and abs(job.dst_y - job.src_y) > LINK_EPS:
+            continue
         job.route(boxes, ledger, roomy, direct_only=True)
+        if job.points is None:
+            deferred.add(group)
     for clear in (roomy, tight_clearance()):
         for job in jobs:
             if job.points is None:
@@ -1140,6 +1234,11 @@ def render_components(components_to_render):
     """Handles parsing and visual painting of given component listings on canvas with slot attributes."""
     canvas = app_state.canvas
     app_state.active_components_list = components_to_render
+    # Repainting the same sheet (a refresh, an undo, a device update) keeps whatever of
+    # the selection is still on it; another sheet starts with nothing selected.
+    kept = [app_state.canvas_comp_map.get(t) for t in app_state.selected_canvas_tags
+            if t != app_state.selected_canvas_tag]
+    kept.append(app_state.canvas_comp_map.get(app_state.selected_canvas_tag))
     app_state.selected_canvas_tag = None
     app_state.selected_canvas_tags.clear()
 
@@ -1160,9 +1259,6 @@ def render_components(components_to_render):
     meta_scale = sheet_meta_scale(components_to_render)
     app_state.meta_scale = meta_scale
 
-    max_reached_x = 1000
-    max_reached_y = 1000
-
     for comp in components_to_render:
         meta_prop = comp.find("prop[@name='meta']")
         meta_str = meta_prop.get("val") if meta_prop is not None else None
@@ -1175,22 +1271,26 @@ def render_components(components_to_render):
         app_state.canvas_comp_map[comp_tag] = comp
         app_state.canvas_tag_by_element[comp] = comp_tag
 
-        box_width, box_height, slot_offsets = draw_component_box(
+        _, _, slot_offsets = draw_component_box(
             canvas, comp, coord_x, coord_y, fonts, linked_slot_index,
             ("component", comp_tag), outline_tags=("block_outline",)
         )
         app_state.component_slot_offsets[comp_tag] = slot_offsets
 
-        if coord_x + box_width > max_reached_x:
-            max_reached_x = coord_x + box_width
-        if coord_y + box_height > max_reached_y:
-            max_reached_y = coord_y + box_height
-
     draw_links()
-    apply_sheet_zoom(fonts, max_reached_x + 200, max_reached_y + 200)
+    apply_sheet_zoom(fonts)
+
+    on_sheet = set(components_to_render)
+    for element in kept:
+        if element is not None and element in on_sheet:
+            tag = app_state.canvas_tag_by_element[element]
+            app_state.selected_canvas_tags.add(tag)
+            app_state.selected_canvas_tag = tag
+    for tag in app_state.selected_canvas_tags:
+        canvas.itemconfig(f"{tag} && block_outline", outline="#1f4e79", width=3)
 
 
-def apply_sheet_zoom(fonts, extent_x, extent_y):
+def apply_sheet_zoom(fonts):
     """Scales the finished sheet to app_state.zoom and sizes the canvas to match.
 
     Painting and routing both work at 1:1, because that is where every metric here is
@@ -1212,10 +1312,46 @@ def apply_sheet_zoom(fonts, extent_x, extent_y):
 
         scale_knob_ends(zoom)
 
-    canvas.config(scrollregion=(0, 0, extent_x * zoom, extent_y * zoom))
+    fit_sheet_region()
+
+
+def fit_sheet_region():
+    """Sets the scrollable area: the whole sheet, plus whatever hangs off its edges.
+
+    The sheet itself never grows (see sheet_extent). A box near the right or bottom
+    edge can still reach past it, or a wire loop past the top, so the scrollable area
+    takes those in too, with SHEET_MARGIN_GRIDS of grey beyond them.
+    """
+    canvas = app_state.canvas
+    zoom = app_state.zoom
+    margin = SHEET_MARGIN_GRIDS * app_state.GRID_SIZE
+    width, height = sheet_extent()
+    x1, y1, x2, y2 = 0, 0, width, height
+    bounds = sheet_content_bounds()
+    if bounds is not None:
+        if bounds[0] < 0:
+            x1 = bounds[0] - margin
+        if bounds[1] < 0:
+            y1 = bounds[1] - margin
+        if bounds[2] > width:
+            x2 = bounds[2] + margin
+        if bounds[3] > height:
+            y2 = bounds[3] + margin
+    canvas.config(scrollregion=(x1 * zoom, y1 * zoom, x2 * zoom, y2 * zoom))
     draw_grid()
 
 
+def scroll_view_to(x, y):
+    """Scrolls so canvas point (x, y) sits at the view's top-left, as near as the sheet allows."""
+    canvas = app_state.canvas
+    region = str(canvas.cget("scrollregion")).split()
+    if len(region) != 4:
+        return
+    x1, y1, x2, y2 = (float(v) for v in region)
+    canvas.xview_moveto(max(0.0, (x - x1) / ((x2 - x1) or 1.0)))
+    canvas.yview_moveto(max(0.0, (y - y1) / ((y2 - y1) or 1.0)))
+
+
 def scale_knob_ends(zoom):
     """Brings the cached knob-to-knob ends onto the painted scale.
 
@@ -1237,6 +1373,7 @@ def redraw_links():
     zoom = app_state.zoom
     if zoom == 1.0:
         draw_links()
+        fit_sheet_region()   # a box dragged over the edge brings the grey past it into view
         return
 
     canvas = app_state.canvas
@@ -1245,6 +1382,7 @@ def redraw_links():
     canvas.scale("component", 0, 0, zoom, zoom)
     canvas.scale("link", 0, 0, zoom, zoom)
     scale_knob_ends(zoom)
+    fit_sheet_region()
 
 
 def zoom_step(direction):
@@ -1307,14 +1445,8 @@ def fit_zoom_level():
     if bounds is None:
         return None
 
-    # A canvas cannot scroll left of or above its origin, so anything out there cannot
-    # be brought into view however far the sheet is zoomed out. The fit is measured from
-    # the origin in that case rather than from the content's own corner.
-    left = max(0.0, bounds[0])
-    top = max(0.0, bounds[1])
-
-    content_width = bounds[2] - left
-    content_height = bounds[3] - top
+    content_width = bounds[2] - bounds[0]
+    content_height = bounds[3] - bounds[1]
     view_width = canvas.winfo_width() - 2 * FIT_PADDING
     view_height = canvas.winfo_height() - 2 * FIT_PADDING
 
@@ -1411,59 +1543,358 @@ def column_gap_for(run_count):
                + max(0, run_count - 1) * clear.lane)
 
 
-def plan_columns(columns, column_keys, demand, scale):
+LAYOUT_SWEEPS = 4   # down-and-up passes plan_columns makes to straighten the wires
+
+
+def plan_columns(columns, column_keys, demand, scale, wires=()):
     """Walks the columns at `scale` px per meta step, without touching the DOM.
 
     Returns (placements, clamped) where a placement is (item, step_x, step_y). Boxes are
     positioned in pixels exactly as they will be painted, then divided down into meta
     steps, so a coarser scale reaches further across the sheet for the same 0..255 range.
+
+    Column x positions come from widths and wire lanes alone. Heights are then settled
+    to make the wires run as straight as they can: each box is offered the height that
+    lines its slot up with the slot at the other end of its wires (the median, when it
+    has several), and each column is re-sorted by those heights, which also untangles
+    crossings. Boxes keep at least TIDY_GAP apart, but any gap above that is free -- a
+    box drops as far as its wires want. Sweeps run left to right, lining boxes up with
+    what feeds them, and right to left, lining them up with what they feed.
+
+    `wires` are (src_column, dst_column, src_item, src_slot, dst_item, dst_slot), left to
+    right only. One that skips columns stands in each column it crosses as a placeholder
+    box one wire tall, so it is sorted and spaced like the boxes there. The router turns
+    such a wire down beside its source, crossing the columns in between level with its
+    target slot, so a box still in its way is pushed down below it. The last sweep runs
+    right to left so those heights are final when that is checked.
     """
     margin = snap_up(TIDY_MARGIN, scale)
+    clear_end = grid_clearance().end
+    count = len(column_keys)
+
+    # Column x positions depend only on widths and wire lanes, so they come first.
+    column_x = []
+    cursor_x = margin  # the left-most column clears the canvas edge by the margin
+    for index, grid_x in enumerate(column_keys):
+        column_x.append(cursor_x)
+        gap = column_gap_for(demand.get(index, 0))
+        cursor_x = snap_up(cursor_x + max(i["width"] for i in columns[grid_x]) + gap,
+                           scale)
+
+    order = [list(columns[key]) for key in column_keys]
+
+    # A wire skipping columns gets a placeholder in each column it crosses, as tall as a
+    # wire, so it is sorted and spaced among that column's boxes like one of them. That
+    # is what keeps a box from being placed on the wrong side of a wire it then has to
+    # cross. The wire is modelled as a chain through its placeholders, so every link of
+    # the chain joins neighbouring columns.
+    segments = []          # (a, a_slot, b, b_slot), a one column left of b
+    for src_col, dst_col, src, s_slot, dst, d_slot in wires:
+        previous, previous_slot = src, s_slot
+        for index in range(src_col + 1, dst_col):
+            placeholder = {"wire": True, "height": 0, "slot_offsets": {}}
+            order[index].append(placeholder)
+            segments.append((previous, previous_slot, placeholder, None))
+            previous, previous_slot = placeholder, None
+        segments.append((previous, previous_slot, dst, d_slot))
+
+    incoming = {id(item): [] for column in order for item in column}
+    outgoing = {id(item): [] for column in order for item in column}
+    for segment in segments:
+        outgoing[id(segment[0])].append(segment)
+        incoming[id(segment[2])].append(segment)
+
+    def offset(item, slot):
+        if item.get("wire"):
+            return 0
+        return item["slot_offsets"].get(slot, item["height"] / 2)
+
+    def gap(above, below):
+        """Clear space between two neighbours in a column, boxes or wires."""
+        if above.get("wire") and below.get("wire"):
+            return grid_clearance().row
+        if above.get("wire") or below.get("wire"):
+            return clear_end
+        return TIDY_GAP
+
+    top = {}
+    for column in order:                     # starting point: packed from the top
+        cursor_y = margin
+        for item in column:
+            top[id(item)] = cursor_y
+            cursor_y = snap_up(cursor_y + item["height"] + TIDY_GAP, scale)
+
+    def wanted_from_left(item):
+        return [(top[id(a)] + offset(a, a_slot) - offset(item, b_slot), offset(item, b_slot))
+                for a, a_slot, _, b_slot in incoming[id(item)]]
+
+    def wanted_from_right(item):
+        return [(top[id(b)] + offset(b, b_slot) - offset(item, a_slot), offset(item, a_slot))
+                for _, a_slot, b, b_slot in outgoing[id(item)]]
+
+    def place(index, primary, fallback):
+        column = order[index]
+        wanted = {}
+        anchor = {}
+        for item in column:
+            if item.get("loose"):
+                continue
+            heights = sorted(primary(item) or fallback(item))
+            if heights:
+                spot, slot_offset = heights[(len(heights) - 1) // 2]
+            else:
+                spot, slot_offset = top[id(item)], 0
+            wanted[id(item)] = spot
+            anchor[id(item)] = spot + slot_offset   # the wire height it lines up with
+        # Sorted by the wire each box lines up with, so neighbours' wires do not cross;
+        # unlinked boxes stay packed below everything else, in their own order.
+        column.sort(key=lambda i: (bool(i.get("loose")), anchor.get(id(i), 0)))
+
+        rows = sorted(
+            top[id(dst)] + offset(dst, d_slot)
+            for src_col, dst_col, _, _, dst, d_slot in wires
+            if src_col < index < dst_col
+        )
+        floor_y = margin
+        above = None
+        for item in column:
+            if above is not None:
+                floor_y = above_bottom + gap(above, item)
+            if item.get("loose"):
+                spot = snap_up(floor_y, scale)
+            elif item.get("wire"):
+                spot = max(floor_y, wanted[id(item)])   # never stored, so not snapped
+            else:
+                spot = max(snap_up(floor_y, scale), round(wanted[id(item)] / scale) * scale)
+            if not item.get("wire"):
+                # Step below every passing wire the box would still sit across, keeping
+                # the clearance the router wants between a horizontal run and a box.
+                moved = True
+                while moved:
+                    moved = False
+                    for row in rows:
+                        if spot - clear_end < row < spot + item["height"] + clear_end:
+                            spot = snap_up(row + clear_end, scale)
+                            moved = True
+            top[id(item)] = spot
+            above, above_bottom = item, spot + item["height"]
+
+    def lift():
+        """Shifts the whole sheet up until its top-most box sits on the margin again.
+
+        Boxes only ever settle downwards, so without this the sweeps would carry the
+        whole picture further down the sheet each time round.
+        """
+        tops = [top[id(i)] for column in order for i in column
+                if not i.get("loose") and not i.get("wire")]
+        shift = (min(tops, default=margin) - margin) // scale * scale
+        if shift > 0:
+            for key in top:
+                top[key] -= shift
+
+    for _ in range(LAYOUT_SWEEPS):
+        for index in range(count):
+            place(index, wanted_from_left, wanted_from_right)
+        lift()
+        for index in reversed(range(count)):
+            place(index, wanted_from_right, wanted_from_left)
+        lift()
+
     placements = []
     clamped = 0
-    cursor_x = margin  # the left-most column clears the canvas edge by the margin
-
-    for index, grid_x in enumerate(column_keys):
-        column = sorted(columns[grid_x], key=lambda i: i["grid_y"])
-        cursor_y = margin
-
+    for index, column in enumerate(order):
         for item in column:
-            step_x = min(META_COORD_MAX, round(cursor_x / scale))
-            step_y = min(META_COORD_MAX, round(cursor_y / scale))
+            if item.get("wire"):
+                continue
+            step_x = min(META_COORD_MAX, round(column_x[index] / scale))
+            step_y = min(META_COORD_MAX, round(top[id(item)] / scale))
             if step_x == META_COORD_MAX or step_y == META_COORD_MAX:
                 clamped += 1
             placements.append((item, step_x, step_y))
-
-            # Clear the box just placed by at least TIDY_GAP, then step to the next
-            # whole meta step so the box still lands on a storable position.
-            cursor_y = snap_up(cursor_y + item["height"] + TIDY_GAP, scale)
-
-        gap = column_gap_for(demand.get(index, 0))
-        cursor_x = snap_up(cursor_x + max(i["width"] for i in column) + gap, scale)
-
     return placements, clamped
 
 
+def sheet_link_pairs(components):
+    """(source, src_slot, target, dst_slot) for each link between two components here."""
+    root = app_state.xml_root_element
+    pairs = []
+    if root is None:
+        return pairs
+    links_elem = root.find("links")
+    if links_elem is None:
+        return pairs
+
+    comp_by_path = {get_component_path(comp, root): comp for comp in components}
+    for link in links_elem.findall("link"):
+        from_ref, to_ref = link.get("from", ""), link.get("to", "")
+        if "." not in from_ref or "." not in to_ref:
+            continue
+        src_path, src_slot = from_ref.rsplit(".", 1)
+        dst_path, dst_slot = to_ref.rsplit(".", 1)
+        src = comp_by_path.get(src_path)
+        dst = comp_by_path.get(dst_path)
+        if src is not None and dst is not None and src is not dst:
+            pairs.append((src, src_slot, dst, dst_slot))
+    return pairs
+
+
+def flow_columns(items, pairs):
+    """Lays linked boxes out as a left-to-right signal flow, in as few columns as it can.
+
+    Returns a list of columns, each listed top to bottom. A box goes one column right of
+    the furthest box feeding it, so the column count is the length of the longest chain
+    and cannot be smaller. A box nothing feeds is then pulled right to sit just before
+    the first box it feeds, which keeps its wires short. Feedback loops are cut where
+    they run backwards relative to the boxes' current positions, so a loop keeps the
+    direction the author drew it in.
+    """
+    def position(item):
+        return item["grid_x"], item["grid_y"], item["comp"].get("name", "")
+
+    item_of = {item["comp"]: item for item in items}
+    succs = {id(item): [] for item in items}
+    for src, _, dst, _ in pairs:
+        if item_of[dst] not in succs[id(item_of[src])]:
+            succs[id(item_of[src])].append(item_of[dst])
+    for out in succs.values():
+        out.sort(key=position)
+
+    # Depth-first from the left-most boxes; an edge back onto the current path is
+    # feedback and is left out of the layering.
+    forward = {id(item): [] for item in items}
+    state = {}                       # id -> 1 on the current path, 2 finished
+    for start in sorted(items, key=position):
+        if id(start) in state:
+            continue
+        state[id(start)] = 1
+        stack = [(start, iter(succs[id(start)]))]
+        while stack:
+            node, pending = stack[-1]
+            nxt = next(pending, None)
+            if nxt is None:
+                state[id(node)] = 2
+                stack.pop()
+            elif state.get(id(nxt)) == 1:
+                continue             # feedback
+            else:
+                forward[id(node)].append(nxt)
+                if id(nxt) not in state:
+                    state[id(nxt)] = 1
+                    stack.append((nxt, iter(succs[id(nxt)])))
+
+    preds = {id(item): [] for item in items}
+    for item in items:
+        for nxt in forward[id(item)]:
+            preds[id(nxt)].append(item)
+
+    # Longest path from the sources, in topological order.
+    layer = {}
+    indegree = {id(item): len(preds[id(item)]) for item in items}
+    ready = sorted((i for i in items if not indegree[id(i)]), key=position)
+    order = []
+    while ready:
+        node = ready.pop(0)
+        order.append(node)
+        layer[id(node)] = max((layer[id(p)] + 1 for p in preds[id(node)]), default=0)
+        for nxt in forward[id(node)]:
+            indegree[id(nxt)] -= 1
+            if not indegree[id(nxt)]:
+                ready.append(nxt)
+    for node in reversed(order):
+        if not preds[id(node)] and forward[id(node)]:
+            layer[id(node)] = min(layer[id(n)] for n in forward[id(node)]) - 1
+
+    # Into any one box, wires from further left must arrive at higher slots than wires
+    # from nearer columns. A long wire crosses the columns in between level with its
+    # slot, so a nearer box feeding a slot above it would have to cross it on the way.
+    # Only boxes nothing feeds are free to move, so they are held back far enough left.
+    feeders = {}                     # id(target) -> [(slot height, source)]
+    for src, _, dst, d_slot in pairs:
+        src_item, dst_item = item_of[src], item_of[dst]
+        if dst_item in forward[id(src_item)]:
+            feeders.setdefault(id(dst_item), []).append(
+                (dst_item["slot_offsets"].get(d_slot, 0), src_item))
+    sources = [n for n in order if not preds[id(n)] and forward[id(n)]]
+    changed = True
+    while changed:
+        changed = False
+        for node in sources:
+            limit = layer[id(node)]
+            for target_id, fed in feeders.items():
+                own = [height for height, src in fed if src is node]
+                for height, other in fed:
+                    if other is not node and own and height > min(own):
+                        limit = min(limit, layer[id(other)])
+            limit = max(0, limit)
+            if limit < layer[id(node)]:
+                layer[id(node)] = limit
+                changed = True
+
+    columns = [[] for _ in range(max(layer.values(), default=-1) + 1)]
+    for item in sorted(items, key=lambda i: (i["grid_y"], i["grid_x"])):
+        columns[layer[id(item)]].append(item)
+
+    # Order each column after the boxes feeding it, so wires run as level as they can;
+    # then once back right to left, settling the boxes nothing feeds beside the boxes
+    # they feed. Ties keep the author's top-to-bottom order.
+    row = {}
+
+    def settle(column, neighbours):
+        def key(item):
+            near = [row[id(n)] for n in neighbours(item) if id(n) in row]
+            return sum(near) / len(near) if near else row.get(id(item), float("inf"))
+        column.sort(key=key)
+        for index, item in enumerate(column):
+            row[id(item)] = index
+
+    for column in columns:
+        settle(column, lambda i: preds[id(i)])
+    for column in reversed(columns):
+        settle(column, lambda i: forward[id(i)] if not preds[id(i)] else ())
+    return columns
+
+
 def build_layout_items(components):
     """Column-grouped layout items for a sheet, measured rather than read off the canvas.
 
-    Returns (columns, column_keys, demand). Called both while painting -- to settle the
-    scale before anything is drawn -- and by tidy, so the two agree.
+    Returns (columns, column_keys, demand, wires), each column listed top to bottom.
+    Linked boxes are arranged by flow_columns. A box with no link to another box on this
+    sheet -- unlinked, or linked only off-sheet -- goes to the bottom of the first
+    column, alphabetically by name.
+    Called both while painting -- to settle the scale before anything is drawn -- and by
+    tidy, so the two agree.
     """
     fonts = sheet_fonts()
     linked_slot_index = build_linked_slot_index()
+    pairs = sheet_link_pairs(components)
+    on_sheet = {comp for src, _, dst, _ in pairs for comp in (src, dst)}
 
-    columns = {}
+    linked = []
+    loose = []
     for comp in components:
         meta_prop = comp.find("prop[@name='meta']")
         meta_val = meta_prop.get("val") if meta_prop is not None else None
         grid_x, grid_y = read_meta_property(meta_val)[:2]
-        width, height = measure_box(comp, fonts, linked_slot_index)[:2]
-        columns.setdefault(grid_x, []).append({
+        width, height, slot_rows = measure_box(comp, fonts, linked_slot_index)[:3]
+        item = {
             "comp": comp, "meta": meta_val,
             "grid_x": grid_x, "grid_y": grid_y,
             "width": width, "height": height,
-        })
+            # Where each slot's wire meets the box, as painted by draw_component_box.
+            "slot_offsets": {
+                raw_name: HEADER_HEIGHT + row * SLOT_ROW_HEIGHT + SLOT_ROW_HEIGHT // 2
+                for row, (raw_name, _, _) in enumerate(slot_rows)
+            },
+        }
+        (linked if comp in on_sheet else loose).append(item)
+
+    columns = dict(enumerate(flow_columns(linked, pairs)))
+    if loose:
+        loose.sort(key=lambda i: (i["comp"].get("name", "").casefold(),
+                                  i["comp"].get("name", "")))
+        for item in loose:
+            item["loose"] = True     # plan_columns keeps these packed at the bottom
+        columns.setdefault(0, []).extend(loose)
 
     column_keys = sorted(columns)
     column_of_comp = {
@@ -1471,7 +1902,42 @@ def build_layout_items(components):
         for index, key in enumerate(column_keys)
         for item in columns[key]
     }
-    return columns, column_keys, wire_demand_by_boundary(column_of_comp)
+    return (columns, column_keys, wire_demand_by_boundary(column_of_comp),
+            forward_wires(columns, column_of_comp))
+
+
+def forward_wires(columns, column_of_comp):
+    """Every wire running left to right between two columns, for plan_columns.
+
+    Each is (src_column, dst_column, src_item, src_slot, dst_item, dst_slot). Feedback
+    wires, running right to left, are left out: they go over or under the boxes anyway.
+    """
+    wires = []
+    root = app_state.xml_root_element
+    links_elem = root.find("links") if root is not None else None
+    if links_elem is None:
+        return wires
+
+    item_by_path = {
+        get_component_path(item["comp"], root): item
+        for column in columns.values()
+        for item in column
+    }
+    for link in links_elem.findall("link"):
+        from_ref, to_ref = link.get("from", ""), link.get("to", "")
+        if "." not in from_ref or "." not in to_ref:
+            continue
+        src_path, src_slot = from_ref.rsplit(".", 1)
+        dst_path, dst_slot = to_ref.rsplit(".", 1)
+        src_item = item_by_path.get(src_path)
+        dst_item = item_by_path.get(dst_path)
+        if src_item is None or dst_item is None:
+            continue
+        src_col = column_of_comp[src_item["comp"]]
+        dst_col = column_of_comp[dst_item["comp"]]
+        if dst_col > src_col:
+            wires.append((src_col, dst_col, src_item, src_slot, dst_item, dst_slot))
+    return wires
 
 
 def sheet_meta_scale(components):
@@ -1485,22 +1951,24 @@ def sheet_meta_scale(components):
     if not components:
         return grid_size
 
-    columns, column_keys, demand = build_layout_items(components)
+    columns, column_keys, demand, wires = build_layout_items(components)
     if not column_keys:
         return grid_size
 
     for multiplier in range(1, META_SCALE_MAX + 1):
-        _, clamped = plan_columns(columns, column_keys, demand, grid_size * multiplier)
+        _, clamped = plan_columns(columns, column_keys, demand, grid_size * multiplier,
+                                  wires)
         if not clamped:
             return grid_size * multiplier
     return grid_size * META_SCALE_MAX
 
 
 def tidy_layout():
-    """Re-flows the open sheet so no two boxes overlap, preserving the author's columns.
+    """Re-flows the open sheet as a left-to-right signal flow with no two boxes overlapping.
 
-    Boxes keep their column grouping and their top-to-bottom order; only the spacing is
-    rebuilt, from the sizes actually painted. Every placement lands on a whole meta step,
+    Columns follow the wiring (see flow_columns), using as few as the longest chain of
+    links allows; spacing is rebuilt from the sizes actually painted. Boxes with no link to another box on the
+    sheet are gathered at the bottom of the first column, A to Z. Every placement lands on a whole meta step,
     the first column and row clear the canvas edge by at least TIDY_MARGIN, and
     neighbours are left at least TIDY_GAP apart, widened where wires need lanes. A sheet
     too wide for meta's 0..255 range is laid out at a coarser multiple of the grid rather
@@ -1511,12 +1979,12 @@ def tidy_layout():
         return 0, 0
 
     components = list(app_state.canvas_comp_map.values())
-    columns, column_keys, demand = build_layout_items(components)
+    columns, column_keys, demand, wires = build_layout_items(components)
     if not column_keys:
         return 0, 0
 
     scale = sheet_meta_scale(components)
-    placements, clamped = plan_columns(columns, column_keys, demand, scale)
+    placements, clamped = plan_columns(columns, column_keys, demand, scale, wires)
 
     changes = [
         (item["comp"], item["meta"], step_x, step_y)