fix: repair tree editor drag-to-reorder with 6 bug fixes

- Grip-only drag initiation (prevents conflict with click-to-select)
- onDragEnd on each draggable item (clears ghost state after failed drops)
- Trailing drop zone after last child (enables drop-to-last-position)
- Suppress cross-parent drag indicators (no misleading visual feedback)
- onDragLeave handler to clear drop indicators when cursor exits
- Source parent tracking threaded through component tree

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

frontend/src/components/tree-editor/NodeList.tsx

@@ -1,4 +1,4 @@
-import { useState } from 'react'
+import { useState, useRef } from 'react'
 import {
   Plus,
   Pencil,
@@ -34,7 +34,10 @@ interface NodeListItemProps {
   onDragStart: (e: React.DragEvent, nodeId: string, parentId: string | null, index: number) => void
   onDragOver: (e: React.DragEvent, parentId: string | null, index: number) => void
   onDrop: (e: React.DragEvent, parentId: string | null, index: number) => void
+  onDragEnd: () => void
+  onDragLeave: (e: React.DragEvent) => void
   dragOverTarget: { parentId: string | null; index: number } | null
+  dragSourceParentId: string | null | undefined
   /** Array of booleans indicating which ancestor levels should show continuing lines */
   ancestorLines?: boolean[]
 }
@@ -53,11 +56,15 @@ function NodeListItem({
   onDragStart,
   onDragOver,
   onDrop,
+  onDragEnd,
+  onDragLeave,
   dragOverTarget,
+  dragSourceParentId,
   ancestorLines = []
 }: NodeListItemProps) {
   const { selectedNodeId, selectNode, validationErrors } = useTreeEditorStore()
   const [isCollapsed, setIsCollapsed] = useState(false)
+  const gripInitiated = useRef(false)
   const isSelected = selectedNodeId === node.id
   const isRootNode = node.id === 'root'
   const nodeErrors = validationErrors.filter(e => e.nodeId === node.id && e.severity === 'error')
@@ -79,7 +86,9 @@ function NodeListItem({
   }
 
   const isDragTarget =
-    dragOverTarget?.parentId === parentId && dragOverTarget?.index === index
+    dragOverTarget?.parentId === parentId &&
+    dragOverTarget?.index === index &&
+    (dragSourceParentId === undefined || dragSourceParentId === parentId)
 
   const nodeTypeIcons: Record<NodeType, React.ReactNode> = {
     decision: <HelpCircle className="h-4 w-4" />,
@@ -148,8 +157,19 @@ function NodeListItem({
 
       <div
         draggable={node.id !== 'root'}
-        onDragStart={(e) => onDragStart(e, node.id, parentId, index)}
+        onDragStart={(e) => {
+          if (!gripInitiated.current) {
+            e.preventDefault()
+            return
+          }
+          onDragStart(e, node.id, parentId, index)
+        }}
+        onDragEnd={() => {
+          gripInitiated.current = false
+          onDragEnd()
+        }}
         onDragOver={(e) => onDragOver(e, parentId, index)}
+        onDragLeave={onDragLeave}
         onDrop={(e) => onDrop(e, parentId, index)}
         onClick={() => selectNode(node.id)}
         className={cn(
@@ -187,7 +207,11 @@ function NodeListItem({
 
         {/* Drag handle */}
         {node.id !== 'root' && (
-          <GripVertical className="h-4 w-4 cursor-grab text-muted-foreground opacity-0 group-hover:opacity-100" />
+          <GripVertical
+            className="h-4 w-4 cursor-grab text-muted-foreground opacity-0 group-hover:opacity-100"
+            onMouseDown={() => { gripInitiated.current = true }}
+            onMouseUp={() => { gripInitiated.current = false }}
+          />
         )}
         {node.id === 'root' && <div className="w-4" />}
 
@@ -336,11 +360,35 @@ function NodeListItem({
             onDragStart={onDragStart}
             onDragOver={onDragOver}
             onDrop={onDrop}
+            onDragEnd={onDragEnd}
+            onDragLeave={onDragLeave}
             dragOverTarget={dragOverTarget}
+            dragSourceParentId={dragSourceParentId}
             ancestorLines={childAncestorLines}
           />
         )
       })}
+
+      {/* Trailing drop zone — allows dropping after the last child */}
+      {!isCollapsed && hasChildren && (() => {
+        const trailingIndex = node.children!.length
+        const isTrailingTarget =
+          dragOverTarget?.parentId === node.id &&
+          dragOverTarget?.index === trailingIndex &&
+          (dragSourceParentId === undefined || dragSourceParentId === node.id)
+        return (
+          <div
+            className={cn(
+              'h-2 rounded-full mx-2 transition-colors',
+              isTrailingTarget ? 'bg-primary' : 'bg-transparent'
+            )}
+            style={{ marginLeft: `${(depth + 1) * 20 + 8}px` }}
+            onDragOver={(e) => onDragOver(e, node.id, trailingIndex)}
+            onDragLeave={onDragLeave}
+            onDrop={(e) => onDrop(e, node.id, trailingIndex)}
+          />
+        )
+      })()}
     </>
   )
 }
@@ -402,9 +450,20 @@ export function NodeList() {
     // Don't allow dropping on itself or its descendants
     if (dragState.nodeId === parentId) return
 
+    // Suppress cross-parent drag indicator (not supported yet)
+    if (dragState.parentId !== parentId) return
+
     setDragOverTarget({ parentId, index })
   }
 
+  const handleDragLeave = (e: React.DragEvent) => {
+    // Only clear if leaving to an element outside the current target
+    const relatedTarget = e.relatedTarget as HTMLElement | null
+    if (!relatedTarget || !e.currentTarget.contains(relatedTarget)) {
+      setDragOverTarget(null)
+    }
+  }
+
   const handleDrop = (
     e: React.DragEvent,
     targetParentId: string | null,
@@ -441,7 +500,7 @@ export function NodeList() {
   }
 
   return (
-    <div className="rounded-lg border border-border bg-card" onDragEnd={handleDragEnd}>
+    <div className="rounded-lg border border-border bg-card">
       <div className="flex items-center justify-between border-b border-border p-3">
         <h2 className="text-sm font-semibold text-card-foreground">Nodes</h2>
         <button
@@ -471,7 +530,10 @@ export function NodeList() {
           onDragStart={handleDragStart}
           onDragOver={handleDragOver}
           onDrop={handleDrop}
+          onDragEnd={handleDragEnd}
+          onDragLeave={handleDragLeave}
           dragOverTarget={dragOverTarget}
+          dragSourceParentId={dragState?.parentId}
         />
       </div>