feat(app): deleting a studyset can be undone for a moment

Decided 2026-10-01. Deleting still asks first; once confirmed, the studyset
leaves the library at once and "Studyset deleted" offers Undo for three
seconds. Only when the time to undo is over (four seconds: the message's
three and a moment for a late tap) are its cards and then the studyset
blanked and deleted, since blanking can't be undone (ADR 0002, rule 2). A
deletion still waiting when the library closes (the app quits, the person
changes) is dropped, and the studyset stays.

The library's bloc keeps the waiting deletions; the event that ends the
time to undo is private, so no screen can skip it. AGENTS.md now describes
the library's shared widgets, the sheets (and why a sheet that asks before
discarding can't be swiped away), the order of studysets, and this Undo.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

halkyon_learn_flutter/test/feature/studyset/bloc/studyset_list_bloc_test.dart

@@ -25,11 +25,6 @@ void main() {
     studysets = StudysetRepository(gateway);
     cardGateway = FakeCardGateway();
     cards = CardRepository(cardGateway);
-    bloc = StudysetListBloc(
-      studysets,
-      cards,
-      ReviewRepository(FakeReviewGateway()),
-    );
   });
   tearDown(() async {
     await bloc.close();
@@ -37,6 +32,16 @@ void main() {
     await cards.dispose();
   });
 
+  /// Opens the library, where a deletion can be undone for [undoWindow].
+  void start({Duration undoWindow = const Duration(milliseconds: 1)}) {
+    bloc = StudysetListBloc(
+      studysets,
+      cards,
+      ReviewRepository(FakeReviewGateway()),
+      undoWindow: undoWindow,
+    )..add(const StudysetListEventStarted());
+  }
+
   /// The next state that lists [titles], each studyset with its progress.
   Future<StudysetListStateLoaded> listing(List<String> titles) async {
     bool matches(StudysetListState state) =>
@@ -62,7 +67,7 @@ void main() {
     final german = await studysets.create(StudysetContent.titled('German'));
     await cards.create(japanese, newCard());
 
-    bloc.add(const StudysetListEventStarted());
+    start();
     // German changed last: it was created after Japanese.
     final state = await listing(['German', 'Japanese']);
 
@@ -77,7 +82,7 @@ void main() {
     final german = await studysets.create(StudysetContent.titled('German'));
     await studysets.star(german);
 
-    bloc.add(const StudysetListEventStarted());
+    start();
 
     await listing(['German', 'Japanese']);
   });
@@ -108,7 +113,7 @@ void main() {
           updatedAt: at(9),
         );
 
-      bloc.add(const StudysetListEventStarted());
+      start();
 
       await listing(['Starred', 'Later', 'Earlier', 'Old']);
     },
@@ -122,13 +127,13 @@ void main() {
       parentId: 'studyset-1',
     );
 
-    bloc.add(const StudysetListEventStarted());
+    start();
 
     await listing(['Japanese']);
   });
 
   test('follows what is created, and the progress as cards change', () async {
-    bloc.add(const StudysetListEventStarted());
+    start();
     await listing([]);
 
     bloc.add(
@@ -148,7 +153,7 @@ void main() {
 
   test("changes a studyset's content", () async {
     final id = await studysets.create(StudysetContent.titled('Japanese'));
-    bloc.add(const StudysetListEventStarted());
+    start();
     await listing(['Japanese']);
 
     bloc.add(
@@ -160,7 +165,7 @@ void main() {
 
   test('says so when the studysets cannot be read, then recovers', () async {
     gateway.failReads = Exception('Disk unavailable');
-    bloc.add(const StudysetListEventStarted());
+    start();
 
     await expectLater(bloc.stream, emits(const StudysetListStateFailed()));
     gateway.failReads = null;
@@ -171,7 +176,7 @@ void main() {
 
   test('stars a studyset and takes the star away', () async {
     await studysets.create(StudysetContent.titled('Japanese'));
-    bloc.add(const StudysetListEventStarted());
+    start();
     final studyset = (await listing(['Japanese'])).studysets.single;
 
     bloc.add(StudysetListEventStarToggled(studyset));
@@ -193,7 +198,7 @@ void main() {
 
   test('archives a studyset and takes it out again', () async {
     final id = await studysets.create(StudysetContent.titled('Japanese'));
-    bloc.add(const StudysetListEventStarted());
+    start();
     await listing(['Japanese']);
 
     bloc.add(StudysetListEventArchiveRequested(id));
@@ -210,16 +215,48 @@ void main() {
     );
   });
 
-  test('deletes a studyset with its cards, each blanked first', () async {
+  test('deleting hides a studyset at once, and deletes it with its cards, '
+      'each blanked first, once the time to undo is over', () async {
     final id = await studysets.create(StudysetContent.titled('Japanese'));
     final card = await cards.create(id, newCard());
-    bloc.add(const StudysetListEventStarted());
+    start(undoWindow: const Duration(milliseconds: 50));
     await listing(['Japanese']);
 
     bloc.add(StudysetListEventDeleteRequested(id));
-
     await listing([]);
+    expect(gateway.deleted, isEmpty);
+
+    await Future<void>.delayed(const Duration(milliseconds: 200));
     expect(cardGateway.deletions, ['blank $card', 'delete $card']);
     expect(gateway.deleted, [id]);
+    await listing([]);
+  });
+
+  test('undoing in time keeps the studyset', () async {
+    final id = await studysets.create(StudysetContent.titled('Japanese'));
+    start(undoWindow: const Duration(hours: 1));
+    await listing(['Japanese']);
+
+    bloc.add(StudysetListEventDeleteRequested(id));
+    await listing([]);
+    bloc.add(StudysetListEventDeleteUndone(id));
+
+    await listing(['Japanese']);
+    expect(gateway.deleted, isEmpty);
+  });
+
+  test('a deletion still waiting when the library closes is dropped, and '
+      'the studyset stays', () async {
+    final id = await studysets.create(StudysetContent.titled('Japanese'));
+    start(undoWindow: const Duration(milliseconds: 50));
+    await listing(['Japanese']);
+
+    bloc.add(StudysetListEventDeleteRequested(id));
+    await listing([]);
+    await bloc.close();
+    await Future<void>.delayed(const Duration(milliseconds: 200));
+
+    expect(gateway.deleted, isEmpty);
+    expect(await studysets.all(), hasLength(1));
   });
 }