packages/frontend/tests/app/docs/yorkie-doc-store.test.ts (1)

1657-1667: Consider asserting the post-undo document state, not just canRedo.

After setDocument({ blocks: [block] }) and a single undo(), the document should revert to whatever pre-setDocument state Yorkie tracked (effectively empty). Asserting getDocument().blocks.length === 0 (or similar) here would catch regressions where undo() no-ops silently or only the redo flag flips without actually rewinding the tree.

🤖 Prompt for AI Agents

Verify each finding against the current code and only fix it if needed.

In `@packages/frontend/tests/app/docs/yorkie-doc-store.test.ts` around lines 1657
- 1667, The test currently only checks canRedo after calling yStore.undo();
update the test to also assert the document state actually reverted by calling
yStore.getDocument() and verifying blocks is empty (e.g.,
getDocument().blocks.length === 0 or deep-equal to the initial empty state)
after undo; locate the test using symbols makeBlock, yStore.setDocument,
yStore.undo, yStore.canRedo and add a post-undo assertion that confirms the tree
was rewound, not just the redo flag toggled.

packages/frontend/src/app/docs/yorkie-doc-store.ts (1)

1936-1939: One-way silent fallback loses all undo history — consider snapshotting on flip.

Once useYorkieUndo flips to false, the snapshot stacks are empty and stay that way until the next mutation triggers snapshot(). The user then has no way to undo anything that happened before the failed mergeBlock undo (which arguably is the action they most wanted to revert). Two mitigations worth considering:

1. On flip, push a clone of the current document onto undoStack so at least the post-failure state is recoverable.
2. Surface the flip via onRemoteChange?.() (or a new onUndoModeChange callback) so the host UI can disable the redo button / show a toast, since canUndo()/canRedo() semantics shift the moment the flag flips.

🤖 Prompt for AI Agents

Verify each finding against the current code and only fix it if needed.

In `@packages/frontend/src/app/docs/yorkie-doc-store.ts` around lines 1936 - 1939,
When catching the failed Yorkie undo in the catch block that sets useYorkieUndo
= false (around the mergeBlock undo path), preserve undo history and notify the
UI: push a deep clone of the current document state onto undoStack (or call
snapshot()) immediately before or when flipping useYorkieUndo so users can still
undo the last action, and call onRemoteChange?.() or emit a new
onUndoModeChange(...) callback right after flipping the flag so host UI can
update canUndo()/canRedo() and show a toast/disable redo; update references
around useYorkieUndo, undoStack, snapshot(), mergeBlock, and the
onRemoteChange/onUndoModeChange hooks accordingly.

docs/tasks/active/20260403-intent-preserving-phase9-undo-redo-todo.md (1)

45-47: Tie the Backspace-merge skip to the fallback bug in yorkie-doc-store.ts.

This skip is the same scenario that exposes the empty-snapshot-stack NPE in the undo() fallback path (see review on packages/frontend/src/app/docs/yorkie-doc-store.ts:1927-1948). Until that is fixed, hitting Backspace-merge-undo in the browser with useYorkieUndo: true won't just be "no-op"; it will throw. Worth adding a note here so whoever picks up the production rollout knows the prerequisite.

As per coding guidelines: docs/tasks/active/*-{todo,lessons}.md: For non-trivial tasks, use paired files in docs/tasks/active/.

🤖 Prompt for AI Agents

Verify each finding against the current code and only fix it if needed.

In `@docs/tasks/active/20260403-intent-preserving-phase9-undo-redo-todo.md` around
lines 45 - 47, The Backspace-merge test was skipped because triggering the
undo() fallback path currently throws an empty-snapshot-stack NPE; fix the docs
by adding a note tying that skip to the bug in the undo() fallback and add a
paired lessons file as per the repo guideline. Update the todo entry to
explicitly state: Backspace-merge→undo is blocked until the empty-snapshot-stack
NPE in the undo() fallback (in yorkie-doc-store) is fixed; create the
corresponding docs/tasks/active/*-lessons.md file summarizing the root cause
(empty snapshot stack handling) and the required code change (guard against
empty snapshot stack or ensure snapshot push/pop invariants in the undo()
fallback). Ensure you reference the undo() fallback path and the
empty-snapshot-stack NPE in the new note so the production rollout owner knows
the prerequisite.