fix(dragdrop): unify tree+tab drop zone logic, fix cross-pane transfer

Three fixes for drag-and-drop in split view (#75):

1. Tree drag-to-split: edge zone detection now runs BEFORE the tree/tab
   dispatch, so dragging a sidebar file to the right/bottom edge of a
   single pane correctly creates a split and opens the file in the new
   pane (was: tree check returned early before edge zone logic).

2. Cross-pane transfer consistency: transferTab() now tracks pane count
   before the transfer — if closePane() fires inside transferTabTo()
   (source pane becomes empty), the DOM is rebuilt and the original
   targetTm reference is stale. After close, re-lookup the tab manager
   that actually holds the transferred tab before _renderTabs()+activate.

3. dragdrop.js flicker: onDragLeave() now checks relatedTarget instead
   of unconditionally clearing highlights on every child boundary.

frontend/js/dragdrop.js

@@ -196,10 +196,11 @@ function onDragOver(e) {
 }
 
 function onDragLeave(e) {
-  // Only clear when actually leaving the container, not entering a child
+  // Only clear when actually leaving the container, not entering a child.
+  // Use relatedTarget: if it's still inside #vault-tree, keep highlights.
   const container = document.getElementById('vault-tree');
   if (!container) return;
-  // Use a small delay to avoid flickering when moving between children
+  if (e.relatedTarget && container.contains(e.relatedTarget)) return;
   clearAllHighlights();
 }
 

frontend/js/pane-manager.js

@@ -659,12 +659,29 @@ const PaneManager = {
     const fromTm = this.tabManagers[fromPaneId];
     const toTm = this.tabManagers[toPaneId];
     if (!fromTm || !toTm) return false;
+
+    // Track state BEFORE transfer (closePane may rebuild DOM)
+    const paneCountBefore = this.panes.length;
+
     const ok = fromTm.transferTabTo(tabId, toTm);
-    if (ok) {
-      toTm._renderTabs();
-      toTm.activate(tabId);
-      this._persist();
+    if (!ok) return false;
+
+    // If source pane was auto-closed, DOM was rebuilt — re-lookup target
+    let actualTm = toTm;
+    if (this.panes.length < paneCountBefore) {
+      // Find which pane manager now holds the tab
+      for (const [pid, tm] of Object.entries(this.tabManagers)) {
+        if (tm._tabs.find(t => t.id === tabId)) {
+          actualTm = tm;
+          console.log('[PaneManager] source closed, tab now in pane', pid);
+          break;
+        }
+      }
     }
+
+    actualTm._renderTabs();
+    actualTm.activate(tabId);
+    this._persist();
     return ok;
   },
 
@@ -1060,15 +1077,53 @@ const PaneManager = {
 
       if (!raw) return;
 
-      // Handle tree drag (file from sidebar)
+      // --- Determine drop zone ---
+      const rect = container.getBoundingClientRect();
+      const relX = e.clientX - rect.left;
+      const relY = e.clientY - rect.top;
+      const isSingle = !this.isSplit();
+      const onRightEdge = isSingle && relX > rect.width * 0.72;
+      const onBottomEdge = isSingle && relY > rect.height * 0.72;
+
+      console.log('[PaneManager] drop at', relX.toFixed(0), relY.toFixed(0),
+        'rect', rect.width.toFixed(0), 'x', rect.height.toFixed(0),
+        'isSingle', isSingle, 'paneId', paneId,
+        'onRightEdge', onRightEdge, 'onBottomEdge', onBottomEdge);
+
+      // --- Tree drag (file from sidebar) ---
       if (raw.startsWith('tree:')) {
         try {
           const treeData = JSON.parse(raw.substring(5));
-          if (treeData.vault && treeData.path) {
-            console.log('[PaneManager] tree drop → opening in pane', paneId);
-            const tm = window.PaneManager.getTabManager(paneId);
+          if (!treeData.vault || !treeData.path) return;
+          console.log('[PaneManager] tree drop vault=', treeData.vault, 'path=', treeData.path);
+
+          if (onRightEdge) {
+            console.log('[PaneManager] → tree splitRight');
+            this.splitRight();
+            setTimeout(() => {
+              const newPaneId = this.panes.length - 1;
+              const tm = this.getTabManager(newPaneId);
+              if (tm) {
+                this.setActivePane(newPaneId);
+                tm.openPreview(treeData.vault, treeData.path);
+              }
+            }, 200);
+          } else if (onBottomEdge) {
+            console.log('[PaneManager] → tree splitDown');
+            this.splitDown();
+            setTimeout(() => {
+              const newPaneId = this.panes.length - 1;
+              const tm = this.getTabManager(newPaneId);
+              if (tm) {
+                this.setActivePane(newPaneId);
+                tm.openPreview(treeData.vault, treeData.path);
+              }
+            }, 200);
+          } else {
+            console.log('[PaneManager] → tree open in pane', paneId);
+            const tm = this.getTabManager(paneId);
             if (tm) {
-              window.PaneManager.setActivePane(paneId);
+              this.setActivePane(paneId);
               tm.openPreview(treeData.vault, treeData.path);
             }
           }
@@ -1076,39 +1131,26 @@ const PaneManager = {
         return;
       }
 
-      // Handle tab drag
+      // --- Tab drag ---
       let dragData;
       try { dragData = JSON.parse(raw); } catch (e2) { console.log('[PaneManager] JSON parse error:', e2); return; }
       if (dragData.paneId == null || dragData.tabId == null) { console.log('[PaneManager] missing paneId/tabId'); return; }
 
-      // Recalculate edge zone from drop coordinates
-      const rect = container.getBoundingClientRect();
-      const relX = e.clientX - rect.left;
-      const relY = e.clientY - rect.top;
-      const isSingle = !this.isSplit();
-
-      console.log('[PaneManager] drop at', relX.toFixed(0), relY.toFixed(0),
-        'rect', rect.width.toFixed(0), 'x', rect.height.toFixed(0),
-        'isSingle', isSingle, 'paneId', paneId, 'srcPane', dragData.paneId);
-
-      if (isSingle && relX > rect.width * 0.72) {
+      if (onRightEdge) {
         console.log('[PaneManager] → splitRight');
         this.splitRight();
-        // Transfer after DOM settles
-        const transfer = () => {
+        setTimeout(() => {
           const newPaneId = this.panes.length - 1;
           console.log('[PaneManager] transferring tab', dragData.tabId, 'to pane', newPaneId);
           this.transferTab(dragData.paneId, dragData.tabId, newPaneId);
-        };
-        setTimeout(transfer, 200);
-      } else if (isSingle && relY > rect.height * 0.72) {
+        }, 200);
+      } else if (onBottomEdge) {
         console.log('[PaneManager] → splitDown');
         this.splitDown();
-        const transfer = () => {
+        setTimeout(() => {
           const newPaneId = this.panes.length - 1;
           this.transferTab(dragData.paneId, dragData.tabId, newPaneId);
-        };
-        setTimeout(transfer, 200);
+        }, 200);
       } else if (dragData.paneId !== paneId) {
         console.log('[PaneManager] → cross-pane transfer', dragData.paneId, '→', paneId);
         this.transferTab(dragData.paneId, dragData.tabId, paneId);