fix+feat: blank options, stub card dismiss, collapsible subtrees

- TreeCanvas: strip blank-label options on save so they don't generate
  stubs; also filter them from the unlinked-option add-button list
- AnswerStubCard: collapse type-picker when clicking outside the card
- TreeCanvasNode: add subtree collapse toggle button (ChevronsDownUp icon)
  visible in compact mode when the node has children
- TreeCanvas: track collapsedNodeIds; hide subtree behind a clickable
  "N nodes hidden" pill when collapsed

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

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

@@ -1,4 +1,4 @@
-import { useState } from 'react'
+import { useState, useRef, useEffect } from 'react'
 import { HelpCircle, Zap, CheckCircle, Trash2 } from 'lucide-react'
 import { cn } from '@/lib/utils'
 import type { TreeStructure } from '@/types'
@@ -13,10 +13,24 @@ interface AnswerStubCardProps {
 export function AnswerStubCard({ node, fromOption, onSelectType, onDelete }: AnswerStubCardProps) {
   const [picking, setPicking] = useState(false)
   const [confirming, setConfirming] = useState(false)
+  const cardRef = useRef<HTMLDivElement>(null)
   const label = fromOption || node.title || 'Answer'
 
+  // Collapse picker when clicking outside the card
+  useEffect(() => {
+    if (!picking) return
+    const handleOutsideClick = (e: MouseEvent) => {
+      if (cardRef.current && !cardRef.current.contains(e.target as Node)) {
+        setPicking(false)
+      }
+    }
+    document.addEventListener('mousedown', handleOutsideClick)
+    return () => document.removeEventListener('mousedown', handleOutsideClick)
+  }, [picking])
+
   return (
     <div
+      ref={cardRef}
       className={cn(
         'relative min-w-[180px] max-w-[280px] rounded-xl border-2 border-dashed border-border bg-card/50',
         'transition-all duration-150',

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

@@ -163,6 +163,7 @@ export function TreeCanvas() {
   // ── Local canvas state ──
   const [expandedNodeId, setExpandedNodeId] = useState<string | null>(null)
   const [newNodeIds, setNewNodeIds] = useState<Set<string>>(new Set())
+  const [collapsedNodeIds, setCollapsedNodeIds] = useState<Set<string>>(new Set())
   const [pendingAddKey, setPendingAddKey] = useState<string | null>(null)
   const [pendingLinks, setPendingLinks] = useState<Map<string, PendingLink>>(
     new Map()
@@ -204,26 +205,26 @@ export function TreeCanvas() {
     (nodeId: string, updates: Partial<TreeStructure>) => {
       updateNode(nodeId, updates)
 
-      // For decision nodes: create answer stubs for any option without a next_node_id
+      // For decision nodes: strip blank options, then create answer stubs for any
+      // labelled option that doesn't yet have a linked child
       if (updates.options) {
-        const options = updates.options
+        const options = updates.options.filter((o) => o.label.trim())
         const stubsToCreate: Array<{ opt: typeof options[number]; stubId: string }> = []
 
         options.forEach((opt) => {
-          if (!opt.next_node_id && opt.label.trim()) {
+          if (!opt.next_node_id) {
             const stubId = addNode(nodeId, 'answer')
             updateNode(stubId, { title: opt.label })
             stubsToCreate.push({ opt, stubId })
           }
         })
 
-        if (stubsToCreate.length > 0) {
-          const updatedOptions = options.map((o) => {
-            const stub = stubsToCreate.find((s) => s.opt.id === o.id)
-            return stub ? { ...o, next_node_id: stub.stubId } : o
-          })
-          updateNode(nodeId, { options: updatedOptions })
-        }
+        // Write back: filtered options + any newly assigned next_node_ids
+        const updatedOptions = options.map((o) => {
+          const stub = stubsToCreate.find((s) => s.opt.id === o.id)
+          return stub ? { ...o, next_node_id: stub.stubId } : o
+        })
+        updateNode(nodeId, { options: updatedOptions })
       }
 
       // Resolve pending link for new nodes
@@ -301,6 +302,16 @@ export function TreeCanvas() {
     [duplicateNode]
   )
 
+  // ── Subtree collapse toggle ──
+  const handleToggleSubtreeCollapse = useCallback((nodeId: string) => {
+    setCollapsedNodeIds((prev) => {
+      const next = new Set(prev)
+      if (next.has(nodeId)) next.delete(nodeId)
+      else next.add(nodeId)
+      return next
+    })
+  }, [])
+
   // ── Convert answer stub to a real node type ──
   const handleSelectAnswerType = useCallback(
     (nodeId: string, type: 'decision' | 'action' | 'solution') => {
@@ -416,6 +427,7 @@ export function TreeCanvas() {
     ): React.ReactNode => {
       const isExpanded = expandedNodeId === node.id
       const isNew = newNodeIds.has(node.id)
+      const isSubtreeCollapsed = collapsedNodeIds.has(node.id)
       const nodeChildren = node.children || []
 
       // For decision nodes, order children by option link order
@@ -467,8 +479,9 @@ export function TreeCanvas() {
         node.type === 'decision' && node.options
           ? node.options.filter(
               (opt) =>
-                !opt.next_node_id ||
-                !nodeChildren.find((c) => c.id === opt.next_node_id)
+                opt.label.trim() &&
+                (!opt.next_node_id ||
+                  !nodeChildren.find((c) => c.id === opt.next_node_id))
             )
           : []
 
@@ -512,7 +525,10 @@ export function TreeCanvas() {
               fromOption={optionLabel}
               isExpanded={isExpanded}
               isNew={isNew}
+              hasChildren={nodeChildren.length > 0}
+              isSubtreeCollapsed={isSubtreeCollapsed}
               onToggleExpand={() => handleToggleExpand(node.id)}
+              onToggleSubtreeCollapse={() => handleToggleSubtreeCollapse(node.id)}
               onSave={handleSave}
               onCancelNew={handleCancelNew}
               onDelete={handleDelete}
@@ -571,8 +587,22 @@ export function TreeCanvas() {
             </div>
           )}
 
+          {/* Collapsed subtree pill */}
+          {hasChildren && !isExpanded && isSubtreeCollapsed && (
+            <div className="mt-3 flex flex-col items-center">
+              <div className="h-4 w-px bg-border" />
+              <button
+                type="button"
+                onClick={() => handleToggleSubtreeCollapse(node.id)}
+                className="rounded-full border border-dashed border-border bg-card px-3 py-1 text-[10px] text-muted-foreground font-label hover:border-primary/40 hover:text-foreground transition-colors"
+              >
+                {orderedChildren.length} node{orderedChildren.length !== 1 ? 's' : ''} hidden — click to expand
+              </button>
+            </div>
+          )}
+
           {/* Connector + Children */}
-          {hasChildren && !isExpanded && (
+          {hasChildren && !isExpanded && !isSubtreeCollapsed && (
             <div className="mt-3 flex flex-col items-center w-full">
               {/* Trunk line from card down */}
               <div className="h-4 w-px bg-border" />
@@ -622,8 +652,10 @@ export function TreeCanvas() {
     [
       expandedNodeId,
       newNodeIds,
+      collapsedNodeIds,
       dragOverTarget,
       handleToggleExpand,
+      handleToggleSubtreeCollapse,
       handleSave,
       handleCancelNew,
       handleDelete,

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

@@ -13,6 +13,8 @@ import {
   AlertTriangle,
   ChevronDown,
   ChevronRight,
+  ChevronsDownUp,
+  ChevronsUpDown,
 } from 'lucide-react'
 import { useTreeEditorStore } from '@/store/treeEditorStore'
 import { NodeFormDecision } from './NodeFormDecision'
@@ -27,7 +29,10 @@ interface TreeCanvasNodeProps {
   fromOption?: string
   isExpanded: boolean
   isNew: boolean
+  hasChildren?: boolean
+  isSubtreeCollapsed?: boolean
   onToggleExpand: () => void
+  onToggleSubtreeCollapse?: () => void
   onSave: (nodeId: string, updates: Partial<TreeStructure>) => void
   onCancelNew: (nodeId: string) => void
   onDelete: (nodeId: string) => void
@@ -70,7 +75,10 @@ export function TreeCanvasNode({
   fromOption,
   isExpanded,
   isNew,
+  hasChildren = false,
+  isSubtreeCollapsed = false,
   onToggleExpand,
+  onToggleSubtreeCollapse,
   onSave,
   onCancelNew,
   onDelete,
@@ -262,6 +270,21 @@ export function TreeCanvasNode({
           </span>
         )}
 
+        {/* Subtree collapse toggle — only in compact mode when node has children */}
+        {!isExpanded && hasChildren && onToggleSubtreeCollapse && (
+          <button
+            type="button"
+            onClick={(e) => { e.stopPropagation(); onToggleSubtreeCollapse() }}
+            title={isSubtreeCollapsed ? 'Expand subtree' : 'Collapse subtree'}
+            className="rounded p-0.5 text-muted-foreground/50 hover:bg-accent hover:text-foreground shrink-0"
+          >
+            {isSubtreeCollapsed
+              ? <ChevronsUpDown className="h-3.5 w-3.5" />
+              : <ChevronsDownUp className="h-3.5 w-3.5" />
+            }
+          </button>
+        )}
+
         {/* Expand/collapse chevron */}
         {!isExpanded ? (
           <ChevronRight className="h-3.5 w-3.5 text-muted-foreground shrink-0" />