Fix split zone detection and canvas-edge squeeze behavior

Split zone was unreachable: squeezed between outer squeeze (1/6) and
center (1/3), leaving a 1/6-wide band impossible to target reliably.
Users hitting the tile edge always triggered squeeze instead of split.

- Remove squeeze from inside-tile detection entirely. Split now owns
  the outer 1/3 of each tile edge — large, easy to target. Center
  keeps the inner 1/3. Squeeze remains gap-detection only (second pass).

- Fix canvas-edge squeeze (no adjacent neighbor): instead of bisecting
  the edge tile, wrap the entire remaining tree in a new directional
  split with the dragged tile placed at the outer position, then
  equalize — all tiles end up equal size.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

1 files changed, 26 insertions, 13 deletions

diff --git a/ui/windows/plot_window.py b/ui/windows/plot_window.py
index 95b9fea..bc8823f 100644
--- a/ui/windows/plot_window.py
+++ b/ui/windows/plot_window.py
@@ -599,8 +599,8 @@ class LayoutCanvas(QFrame):
     def _zone_at(self, pos: QPoint) -> Optional[Tuple[int, str]]:
         """
         Returns (pane_idx, zone) where zone is one of:
-          squeeze_left/right/top/bottom  — outer 1/6 of tile OR gap between tiles, insert between
-          split_left/right/top/bottom    — 1/6 to 1/3, bisect tile
+          split_left/right/top/bottom    — outer 1/3 of tile, bisect tile
+          squeeze_left/right/top/bottom  — gap between tiles only (second pass)
           center                         — inner 1/3, swap/move
         """
         dragging_idx = self._drag_tile.pane_index if self._drag_tile else -1
@@ -612,22 +612,16 @@ class LayoutCanvas(QFrame):
             rx = pos.x() - rect.x()
             ry = pos.y() - rect.y()
             w, h = rect.width(), rect.height()
-            sq_w = max(1, w // 6)
             sp_w = max(1, w // 3)
-            sq_h = max(1, h // 6)
             sp_h = max(1, h // 3)
 
-            # X-axis zone
-            if rx < sq_w:           x_zone = "squeeze_left"
-            elif rx < sp_w:         x_zone = "split_left"
-            elif rx > w - sq_w:     x_zone = "squeeze_right"
+            # X-axis zone — outer 1/3 = split, inner 1/3 = center
+            if rx < sp_w:           x_zone = "split_left"
             elif rx > w - sp_w:     x_zone = "split_right"
             else:                   x_zone = "center"
 
             # Y-axis zone
-            if ry < sq_h:           y_zone = "squeeze_top"
-            elif ry < sp_h:         y_zone = "split_top"
-            elif ry > h - sq_h:     y_zone = "squeeze_bottom"
+            if ry < sp_h:           y_zone = "split_top"
             elif ry > h - sp_h:     y_zone = "split_bottom"
             else:                   y_zone = "center"
 
@@ -781,9 +775,28 @@ class LayoutCanvas(QFrame):
                 self._reposition_tiles()
                 self.arrangement_changed.emit()
                 return
-            # No neighbor (canvas edge) — bisect the edge tile outward
+            # No neighbor — insert dragged at outer edge of full layout
+            new_tree = _tree_remove(self._tree, dragged_idx)
+            if new_tree is None:
+                self._reposition_tiles(); return
+            dragged_leaf = {"kind": "leaf", "pane": dragged_idx}
+            if direction == "left":
+                new_tree = {"kind": "hsplit", "ratio": 0.5, "first": dragged_leaf, "second": new_tree}
+            elif direction == "right":
+                new_tree = {"kind": "hsplit", "ratio": 0.5, "first": new_tree, "second": dragged_leaf}
+            elif direction == "top":
+                new_tree = {"kind": "vsplit", "ratio": 0.5, "first": dragged_leaf, "second": new_tree}
+            else:
+                new_tree = {"kind": "vsplit", "ratio": 0.5, "first": new_tree, "second": dragged_leaf}
+            new_tree = _tree_equalize_ratios(new_tree)
+            self._tree = new_tree
+            if self._cfg_ref is not None:
+                self._cfg_ref.tree = new_tree
+            self._reposition_tiles()
+            self.arrangement_changed.emit()
+            return
 
-        # split, or squeeze at canvas edge with no neighbor
+        # split — bisect target tile
         new_tree = _tree_remove(self._tree, dragged_idx)
         if new_tree is None:
             self._reposition_tiles()