diff --git a/frontend/src/views/TreeView.tsx b/frontend/src/views/TreeView.tsx
index c0778282..8147ac29 100644
--- a/frontend/src/views/TreeView.tsx
+++ b/frontend/src/views/TreeView.tsx
@@ -183,7 +183,9 @@ function TreeCanvas({ source, tree, src, toast }: {
 
   // Autosave the view document.
   const firstSave = useRef(true)
-  const saveTimer = useRef<number>()
+  // React 19's types dropped the zero-argument useRef overload, so the timer
+  // id starts as an explicit undefined instead.
+  const saveTimer = useRef<number | undefined>(undefined)
   const saveNow = useCallback(() => {
     const doc: TreeViewDoc = {
       version: 1,
@@ -228,7 +230,7 @@ function TreeCanvas({ source, tree, src, toast }: {
     el.addEventListener('wheel', onWheel, { passive: false })
     return () => el.removeEventListener('wheel', onWheel)
   }, [])
-  const scrollTimer = useRef<number>()
+  const scrollTimer = useRef<number | undefined>(undefined)
   const onScroll = () => {
     const el = scrollRef.current
     if (!el) return
diff --git a/frontend/vite.config.ts b/frontend/vite.config.ts
index e25db006..5aea76c5 100644
--- a/frontend/vite.config.ts
+++ b/frontend/vite.config.ts
@@ -9,9 +9,9 @@ export default defineConfig({
     sourcemap:   false,
     rollupOptions: {
       output: {
-        manualChunks: {
-          plotly: ['plotly.js-dist-min'],
-        },
+        // vite 8 (rolldown) took only the function form; the old object form
+        // died at "manualChunks is not a function".
+        manualChunks: (id: string) => (id.includes('plotly.js-dist-min') ? 'plotly' : undefined),
       },
     },
   },