Add center-zone move (swap); restore center drop zone

- Edge zones (left/right/top/bottom): split target tile equally
- Center zone: swap dragged and target tile positions in BSP tree
  (tree structure preserved, only leaf pane values exchanged)
- Center overlay: green highlight (vs blue for split zones)
- _tree_swap: immutable swap of two leaf pane values

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

1 files changed, 30 insertions, 11 deletions

diff --git a/ui/windows/plot_window.py b/ui/windows/plot_window.py
index 80a1624..615b7ca 100644
--- a/ui/windows/plot_window.py
+++ b/ui/windows/plot_window.py
@@ -189,6 +189,17 @@ def _tree_reindex(node: dict, removed_idx: int) -> dict:
             "second": _tree_reindex(node["second"], removed_idx)}
 
 
+def _tree_swap(node: dict, idx_a: int, idx_b: int) -> dict:
+    """Swap two leaf pane values in-place (preserves tree structure)."""
+    if node["kind"] == "leaf":
+        if node["pane"] == idx_a: return {**node, "pane": idx_b}
+        if node["pane"] == idx_b: return {**node, "pane": idx_a}
+        return node
+    return {**node,
+            "first":  _tree_swap(node["first"],  idx_a, idx_b),
+            "second": _tree_swap(node["second"], idx_a, idx_b)}
+
+
 def _tree_leaf_count(node: dict) -> int:
     if node["kind"] == "leaf":
         return 1
@@ -455,8 +466,10 @@ def _zone_overlay_rect(r: QRect, zone: str) -> QRect:
         return QRect(r.x() + r.width() // 2, r.y(), r.width() - r.width() // 2, r.height())
     if zone == "top":
         return QRect(r.x(), r.y(), r.width(), r.height() // 2)
-    # bottom
-    return QRect(r.x(), r.y() + r.height() // 2, r.width(), r.height() - r.height() // 2)
+    if zone == "bottom":
+        return QRect(r.x(), r.y() + r.height() // 2, r.width(), r.height() - r.height() // 2)
+    # center (move)
+    return r
 
 
 class PaneTile(QFrame):
@@ -592,7 +605,7 @@ class LayoutCanvas(QFrame):
                 return (pane_idx, "top")
             if rel_y > h - zone_h:
                 return (pane_idx, "bottom")
-            return None   # center — no drop zone
+            return (pane_idx, "center")  # center — move/swap
         return None
 
     # ── Drag ─────────────────────────────────────────────────────────────────
@@ -638,12 +651,14 @@ class LayoutCanvas(QFrame):
         if dragged_idx == target_idx or self._tree is None:
             self._reposition_tiles()
             return
-        new_tree = _tree_remove(self._tree, dragged_idx)
-        if new_tree is None:
-            # Only one pane
-            self._reposition_tiles()
-            return
-        new_tree = _tree_equalize_ratios(_tree_insert(new_tree, target_idx, dragged_idx, zone))
+        if zone == "center":
+            new_tree = _tree_swap(self._tree, dragged_idx, target_idx)
+        else:
+            new_tree = _tree_remove(self._tree, dragged_idx)
+            if new_tree is None:
+                self._reposition_tiles()
+                return
+            new_tree = _tree_equalize_ratios(_tree_insert(new_tree, target_idx, dragged_idx, zone))
         self._tree = new_tree
         if self._cfg_ref is not None:
             self._cfg_ref.tree = new_tree
@@ -663,8 +678,12 @@ class LayoutCanvas(QFrame):
         overlay = _zone_overlay_rect(r, zone)
         painter = QPainter(self)
         painter.setRenderHint(QPainter.RenderHint.Antialiasing)
-        painter.setPen(QPen(QColor("#3b82f6"), 2))
-        painter.setBrush(QColor(59, 130, 246, 70))
+        if zone == "center":
+            painter.setPen(QPen(QColor("#22c55e"), 2))
+            painter.setBrush(QColor(34, 197, 94, 60))
+        else:
+            painter.setPen(QPen(QColor("#3b82f6"), 2))
+            painter.setBrush(QColor(59, 130, 246, 70))
         painter.drawRoundedRect(overlay, 4, 4)