Keep an endpoint's measurement so its drag stops relighting the edges

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01H7LwYgJfpkbLCTeiAf8U4A

frontend/src/components/Flow/FlowEditor.tsx

@@ -3,6 +3,7 @@ import {
   BackgroundVariant,
   type Connection,
   type Node as FlowCanvasNode,
+  type NodeChange,
   ReactFlow,
   ReactFlowProvider,
   useNodesState,
@@ -412,6 +413,25 @@ function FlowEditorInner({
     () => placementsFor(flowName),
   )
 
+  // React Flow measures a node once and keeps the size on it. Endpoints are
+  // rebuilt on every drag frame, so unless the measurement is carried over
+  // they arrive unmeasured and React Flow drops the edges attached to them
+  // until it has measured again — remounting those edges, which makes them
+  // pulse as if a value had just landed. Their own drag lit up the canvas.
+  const measured = useRef(new Map<string, { width: number; height: number }>())
+  const trackMeasured = useCallback(
+    (changes: NodeChange<FlowCanvasNode>[]) => {
+      for (const change of changes) {
+        if (change.type !== "dimensions" || !change.dimensions) continue
+        if (isEndpointNode({ id: change.id })) {
+          measured.current.set(change.id, change.dimensions)
+        }
+      }
+      onNodesChange(changes)
+    },
+    [onNodesChange],
+  )
+
   /** Where clicking an endpoint takes you: the thing it stands for. */
   const openEndpoint = useCallback(
     (id: string) => {
@@ -435,17 +455,23 @@ function FlowEditorInner({
   // stored: they join at render, after everything that reads or writes
   // canvasNodes, so an autosave, an undo or a delete cannot reach them.
   // biome-ignore lint/correctness/useExhaustiveDependencies: positions change on every drag frame; the key covers the wiring.
-  const external = useMemo(
-    () =>
-      deriveEndpoints(
-        detail.endpoints ?? [],
-        definitions,
-        flowName,
-        new Map(canvasNodes.map((node) => [node.id, node.position])),
-        moved,
-      ),
-    [detail.endpoints, key, flowName, moved],
-  )
+  const external = useMemo(() => {
+    const built = deriveEndpoints(
+      detail.endpoints ?? [],
+      definitions,
+      flowName,
+      new Map(canvasNodes.map((node) => [node.id, node.position])),
+      moved,
+    )
+    return {
+      ...built,
+      nodes: built.nodes.map((node) => {
+        const size = measured.current.get(node.id)
+        return size ? { ...node, measured: size, ...size } : node
+      }),
+    }
+    // biome-ignore lint/correctness/useExhaustiveDependencies: positions change on every drag frame; the key covers the wiring.
+  }, [detail.endpoints, key, flowName, moved])
 
   // Edges follow from the name bindings, so they are derived, never stored.
   // Kept off `external` deliberately: an endpoint's edges depend on which
@@ -856,7 +882,7 @@ function FlowEditorInner({
       <ReactFlow
         nodes={shownNodes}
         edges={edges}
-        onNodesChange={onNodesChange}
+        onNodesChange={trackMeasured}
         onNodeDrag={(_event, _node, dragged) => {
           // An endpoint's position is ours, not React Flow's, so it only
           // follows the pointer if we move it every frame.