Freeze editor pane during note switch to prevent content flash

When switching notes, React Query's placeholderData kept the previous
note's data flowing, causing a brief flash of old content before the
new editor mounted. Freeze editor pane props while isPlaceholderData
is true so the current editor stays in place until the new note loads.

Author: ChristianChiarulli

src/features/shell/use-shell-controller.ts

@@ -926,7 +926,7 @@ export function useShellController() {
   const latestRef = useRef(currentHandlers);
   latestRef.current = currentHandlers;
 
-  const editorPaneProps = useMemo(
+  const nextEditorPaneProps = useMemo(
     () => ({
       archivedAt: currentNote?.archivedAt ?? null,
       deletedAt: currentNote?.deletedAt ?? null,
@@ -1019,6 +1019,16 @@ export function useShellController() {
     ],
   );
 
+  // Freeze editor pane props while React Query is showing placeholder data
+  // from the previous note, so the old note's content doesn't flash.
+  const editorPanePropsRef = useRef(nextEditorPaneProps);
+  if (!noteQuery.isPlaceholderData) {
+    editorPanePropsRef.current = nextEditorPaneProps;
+  }
+  const editorPaneProps = noteQuery.isPlaceholderData
+    ? editorPanePropsRef.current
+    : nextEditorPaneProps;
+
   const publishDialogProps = useMemo(
     () => ({
       initialTitle: currentNote?.title ?? "",