refactor(#289 review): extract useScrollRestoreOnSwap so the test guards real code (F2)

The prior test guarded a verbatim MIRROR of the two scroll-restore useLayoutEffect blocks — the reviewer proved removing '&& editor' from the real page-editor.tsx left the test green (a copy, not the original). Extract the wiring into an exported useScrollRestoreOnSwap(pageId, editor, showStatic) hook (the two effects verbatim + useScrollPosition inside; F1 budget logic untouched), call it once from page-editor.tsx (replacing the removed useScrollPosition call + both effects), and rewrite the test to render the REAL hook — deleting the mirror and the false 'regresses in lockstep' comment (F2-doc). Non-vacuity proven: removing '&& editor' from the real hook reddens the guard test. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
2026-07-02 23:29:01 +03:00
parent 963822bd28
commit 293348f9dc
3 changed files with 106 additions and 110 deletions
@@ -1,4 +1,5 @@
-import { useCallback, useEffect, useRef } from "react";
+import { useCallback, useEffect, useLayoutEffect, useRef } from "react";
+import type { Editor } from "@tiptap/react";

 // Throttle interval for persisting the scroll position while the user reads.
 const SAVE_THROTTLE_MS = 250;
@@ -243,3 +244,37 @@ export function useScrollPosition(pageId: string): {

  return { restoreScrollPosition };
 }
+
+/**
+ * Wires `useScrollPosition` to the page editor's static->live swap lifecycle.
+ *
+ * Extracted from PageEditor so the exact restore triggers (their deps and the
+ * post-swap `&& editor` guard) are directly unit-testable rather than mirrored.
+ * Behaviour is unchanged: `restoreScrollPosition` is idempotent, so re-asserting
+ * the same target from either trigger is a no-op.
+ *
+ * @param pageId      the page whose scroll position is persisted/restored.
+ * @param editor      the tiptap editor instance, or `null` until it is ready.
+ * @param showStatic  whether the static (cached) content is still shown.
+ */
+export function useScrollRestoreOnSwap(
+  pageId: string,
+  editor: Editor | null,
+  showStatic: boolean,
+): void {
+  const { restoreScrollPosition } = useScrollPosition(pageId);
+
+  // Restore as early as the static (cached) content is laid out, before paint,
+  // so the reader's position is applied without a visible jump. Aborts itself if
+  // the reader has already started scrolling (handled inside the hook).
+  useLayoutEffect(() => {
+    restoreScrollPosition();
+  }, [restoreScrollPosition]);
+
+  // Re-assert once after the static -> live editor swap in case the swap reset
+  // the window scroll. Idempotent: a no-op when the position is already correct,
+  // and a no-op after the reader has interacted.
+  useLayoutEffect(() => {
+    if (!showStatic && editor) restoreScrollPosition();
+  }, [showStatic, editor, restoreScrollPosition]);
+}