feat: drop cross-pane fonctionnel + drag-to-split (bord droit/bas)

- _setupPaneDropZone: dragover/drop sur .pane-container (fonctionne même si tab bar vide)
- Drag-to-split: déposer un onglet dans les 25% droit → split right, 25% bas → split down
- Indicateurs visuels: zone bleue semi-transparente au drop, tab-bar dashed outline
- stopPropagation dans le drop handler des tabs pour éviter double-fire
- CSS: .pane-drop-zone-indicator, .tab-bar.drop-target, position:relative sur pane-container

frontend/js/pane-manager.js

@@ -390,6 +390,7 @@ function createPaneTabManager(paneId) {
         });
         el.addEventListener('drop', (e) => {
           e.preventDefault();
+          e.stopPropagation(); // Prevent container handler from double-firing
           document.querySelectorAll('.tab-drop-indicator').forEach(d => d.remove());
           try {
             const raw = e.dataTransfer.getData('application/obsigate-tab');
@@ -720,15 +721,99 @@ const PaneManager = {
     }
 
     container.addEventListener('click', (e) => {
-      // Don't capture clicks on tab items (they have their own handler)
       if (e.target.closest('.tab-item')) return;
       this.setActivePane(idx);
     });
 
+    // --- Drop zone handlers on the pane container ---
+    this._setupPaneDropZone(container, idx);
+
     grid.appendChild(container);
     return { id: idx, element: container, width: null };
   },
 
+  /** Add dragover/drop on a pane container for cross-pane transfers + drag-to-split */
+  _setupPaneDropZone(container, paneId) {
+    let _edgeZone = null; // 'right' | 'bottom' | null
+
+    container.addEventListener('dragover', (e) => {
+      const raw = e.dataTransfer.types.includes('application/obsigate-tab');
+      if (!raw) return;
+      e.preventDefault();
+      e.dataTransfer.dropEffect = 'move';
+
+      // Detect edge zones for drag-to-split (only in single-pane mode)
+      const rect = container.getBoundingClientRect();
+      const relX = e.clientX - rect.left;
+      const relY = e.clientY - rect.top;
+      const isSingle = !this.isSplit();
+
+      // Clean up previous indicators
+      container.querySelectorAll('.pane-drop-zone-indicator').forEach(d => d.remove());
+      _edgeZone = null;
+
+      if (isSingle) {
+        if (relX > rect.width * 0.75) {
+          _edgeZone = 'right';
+        } else if (relY > rect.height * 0.75) {
+          _edgeZone = 'bottom';
+        }
+      }
+
+      if (_edgeZone) {
+        // Show edge zone indicator
+        const indicator = document.createElement('div');
+        indicator.className = 'pane-drop-zone-indicator pane-drop-zone-' + _edgeZone;
+        container.appendChild(indicator);
+      } else {
+        // Show tab-bar highlight for cross-pane transfer
+        const tabBar = container.querySelector('.tab-bar');
+        if (tabBar) tabBar.classList.add('drop-target');
+      }
+    });
+
+    container.addEventListener('dragleave', (e) => {
+      container.querySelectorAll('.pane-drop-zone-indicator').forEach(d => d.remove());
+      const tabBar = container.querySelector('.tab-bar');
+      if (tabBar) tabBar.classList.remove('drop-target');
+      _edgeZone = null;
+    });
+
+    container.addEventListener('drop', (e) => {
+      e.preventDefault();
+      container.querySelectorAll('.pane-drop-zone-indicator').forEach(d => d.remove());
+      const tabBar = container.querySelector('.tab-bar');
+      if (tabBar) tabBar.classList.remove('drop-target');
+
+      const raw = e.dataTransfer.getData('application/obsigate-tab');
+      if (!raw) return;
+      try {
+        const dragData = JSON.parse(raw);
+
+        if (_edgeZone === 'right') {
+          // Drag to right edge → split right with this tab
+          this.splitRight();
+          // Transfer the dragged tab to the new pane
+          setTimeout(() => {
+            const newPaneId = this.panes.length - 1;
+            this.transferTab(dragData.paneId, dragData.tabId, newPaneId);
+          }, 100);
+        } else if (_edgeZone === 'bottom') {
+          // Drag to bottom edge → split down with this tab
+          this.splitDown();
+          setTimeout(() => {
+            const newPaneId = this.panes.length - 1;
+            this.transferTab(dragData.paneId, dragData.tabId, newPaneId);
+          }, 100);
+        } else if (dragData.paneId !== paneId) {
+          // Cross-pane transfer (dropped on tab bar or content area of another pane)
+          this.transferTab(dragData.paneId, dragData.tabId, paneId);
+        }
+      } catch (e2) { /* ignore */ }
+      _edgeZone = null;
+    });
+  },
+
   _addPane() {
     const grid = document.querySelector('.pane-grid');
     if (!grid) return;

frontend/style.css

@@ -1264,6 +1264,7 @@ select {
   min-height: 0;
   border: 2px solid transparent;
   transition: border-color 150ms ease;
+  position: relative;
 }
 
 .pane-container.pane-active {
@@ -1317,6 +1318,38 @@ select {
   border: 2px solid var(--accent);
 }
 
+/* --- Drop zone indicators for drag-to-split --- */
+.pane-drop-zone-indicator {
+  position: absolute;
+  z-index: 10;
+  background: var(--accent);
+  opacity: 0.4;
+  pointer-events: none;
+  transition: opacity 100ms ease;
+}
+
+.pane-drop-zone-indicator.pane-drop-zone-right {
+  top: 0;
+  right: 0;
+  width: 25%;
+  height: 100%;
+}
+
+.pane-drop-zone-indicator.pane-drop-zone-bottom {
+  bottom: 0;
+  left: 0;
+  width: 100%;
+  height: 25%;
+}
+
+/* Tab bar drop target highlight */
+.tab-bar.drop-target {
+  background: var(--accent);
+  opacity: 0.15;
+  outline: 2px dashed var(--accent);
+  outline-offset: -2px;
+}
+
 /* Responsive: disable split view below 768px */
 @media (max-width: 768px) {
   .pane-grid {