Palette: drag onto the tree to add inside a component; hover opens branches

A palette drag dropped on a tree row adds the component inside that row's
component, placed under its lowest child. The row under the pointer is
highlighted, and resting on a collapsed branch opens it, loading it from
the device when live without moving the selection or the open sheet.

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

sax_file.py

@@ -16,6 +16,7 @@ from sax_dom import (
     find_component_by_path,
     find_parent_element,
     get_component_path,
+    read_meta_property,
     restore_children,
     snapshot_children,
     update_meta_property,
@@ -936,9 +937,38 @@ def build_config_values(full_type, overrides=None, device_values=None):
     return config_values
 
 
-def add_component(kit_name, type_name, grid_x, grid_y):
-    """Places a new component of kit::type at a grid position. Returns the element."""
-    parent = current_sheet_container()
+def grid_position_below_children(parent):
+    """A spot for a component added without a drop point: under the parent's lowest child.
+
+    Used when a component is placed from the tree, where there is no point on the sheet
+    to drop it at. Lining it up on the left margin below everything already there keeps
+    it clear of the existing boxes in the common case; Tidy sorts out the rest.
+    """
+    min_step = wiresheet.layout_margin(app_state.meta_scale) // app_state.meta_scale
+    lowest = None
+    for child in parent:
+        if child.tag != "comp":
+            continue
+        meta_prop = child.find("prop[@name='meta']")
+        y = read_meta_property(meta_prop.get("val") if meta_prop is not None else None)[1]
+        lowest = y if lowest is None else max(lowest, y)
+
+    if lowest is None:
+        return min_step, min_step
+    return min_step, max(min_step, min(wiresheet.META_COORD_MAX, lowest + TREE_DROP_GAP))
+
+
+# Meta steps between the lowest existing box and one added under it from the tree.
+TREE_DROP_GAP = 8
+
+
+def add_component(kit_name, type_name, grid_x, grid_y, parent=None):
+    """Places a new component of kit::type at a grid position. Returns the element.
+
+    parent is the component it goes inside; None means the sheet that is open.
+    """
+    if parent is None:
+        parent = current_sheet_container()
     if parent is None:
         messagebox.showwarning("Add Component", "Open a SAX file first.")
         return None