feat(learn): the library on shelves by recency, narrowed to a subject (ADR 0041, D8)

The library as Vaia's (the developer's decisions of 10 Nov 2026): the
starred on a shelf of their own, then those studied or changed in the
last 7 days, the last month and before, each under its heading, the
archive last. Recent is the later of a card's last rating
(StudyProgress.lastStudied) and the studyset's own last change. A
dropdown narrows the library to one of the subjects in use, kept on the
device (DeviceSettings.librarySubject). A card names its subject under
its title, with its icon, and a language course its language beside.

Whats-New: The library groups your studysets by when you last used them, and narrows them to one subject.
Whats-New-De: Die Bibliothek ordnet deine Lernsets danach, wann du sie zuletzt genutzt hast, und zeigt auf Wunsch nur ein Fach.

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

apps/learn/halkyon_learn_flutter/lib/l10n/app_de.arb

@@ -2165,5 +2165,11 @@
   "subjectOther": "Sonstiges / unsicher",
   "studysetSheetSubject": "Fach",
   "studysetSheetSubjectNone": "Fach wählen",
-  "subjectPickerTitle": "Fach"
+  "subjectPickerTitle": "Fach",
+  "libraryAllSubjects": "Alle Fächer",
+  "librarySubjectTooltip": "Ein Fach zeigen",
+  "libraryShelfStarred": "Mit Stern",
+  "libraryShelfLastWeek": "Letzte 7 Tage",
+  "libraryShelfLastMonth": "Letzter Monat",
+  "libraryShelfOlder": "Älter"
 }

apps/learn/halkyon_learn_flutter/lib/l10n/app_en.arb

@@ -2656,5 +2656,17 @@
   "studysetSheetSubjectNone": "Choose a subject",
   "@studysetSheetSubjectNone": {"description": "The subject field before one is chosen."},
   "subjectPickerTitle": "Subject",
-  "@subjectPickerTitle": {"description": "The title of the sheet picking a studyset's subject."}
+  "@subjectPickerTitle": {"description": "The title of the sheet picking a studyset's subject."},
+  "libraryAllSubjects": "All subjects",
+  "@libraryAllSubjects": {"description": "The library's subject dropdown while it shows every subject."},
+  "librarySubjectTooltip": "Show one subject",
+  "@librarySubjectTooltip": {"description": "The tooltip of the library's subject dropdown."},
+  "libraryShelfStarred": "Starred",
+  "@libraryShelfStarred": {"description": "The library's heading over the starred studysets."},
+  "libraryShelfLastWeek": "Last 7 days",
+  "@libraryShelfLastWeek": {"description": "The library's heading over the studysets studied or changed in the last 7 days."},
+  "libraryShelfLastMonth": "Last month",
+  "@libraryShelfLastMonth": {"description": "The library's heading over the studysets studied or changed in the last 30 days, before the last 7."},
+  "libraryShelfOlder": "Older",
+  "@libraryShelfOlder": {"description": "The library's heading over the studysets studied or changed longer ago."}
 }

apps/learn/halkyon_learn_flutter/lib/l10n/app_localizations.dart

@@ -12775,6 +12775,42 @@ abstract class AppLocalizations {
   /// In en, this message translates to:
   /// **'Subject'**
   String get subjectPickerTitle;
+
+  /// The library's subject dropdown while it shows every subject.
+  ///
+  /// In en, this message translates to:
+  /// **'All subjects'**
+  String get libraryAllSubjects;
+
+  /// The tooltip of the library's subject dropdown.
+  ///
+  /// In en, this message translates to:
+  /// **'Show one subject'**
+  String get librarySubjectTooltip;
+
+  /// The library's heading over the starred studysets.
+  ///
+  /// In en, this message translates to:
+  /// **'Starred'**
+  String get libraryShelfStarred;
+
+  /// The library's heading over the studysets studied or changed in the last 7 days.
+  ///
+  /// In en, this message translates to:
+  /// **'Last 7 days'**
+  String get libraryShelfLastWeek;
+
+  /// The library's heading over the studysets studied or changed in the last 30 days, before the last 7.
+  ///
+  /// In en, this message translates to:
+  /// **'Last month'**
+  String get libraryShelfLastMonth;
+
+  /// The library's heading over the studysets studied or changed longer ago.
+  ///
+  /// In en, this message translates to:
+  /// **'Older'**
+  String get libraryShelfOlder;
 }
 
 class _AppLocalizationsDelegate

apps/learn/halkyon_learn_flutter/lib/l10n/app_localizations_de.dart

@@ -8170,4 +8170,22 @@ class AppLocalizationsDe extends AppLocalizations {
 
   @override
   String get subjectPickerTitle => 'Fach';
+
+  @override
+  String get libraryAllSubjects => 'Alle Fächer';
+
+  @override
+  String get librarySubjectTooltip => 'Ein Fach zeigen';
+
+  @override
+  String get libraryShelfStarred => 'Mit Stern';
+
+  @override
+  String get libraryShelfLastWeek => 'Letzte 7 Tage';
+
+  @override
+  String get libraryShelfLastMonth => 'Letzter Monat';
+
+  @override
+  String get libraryShelfOlder => 'Älter';
 }

apps/learn/halkyon_learn_flutter/lib/l10n/app_localizations_en.dart

@@ -8097,6 +8097,24 @@ class AppLocalizationsEn extends AppLocalizations {
 
   @override
   String get subjectPickerTitle => 'Subject';
+
+  @override
+  String get libraryAllSubjects => 'All subjects';
+
+  @override
+  String get librarySubjectTooltip => 'Show one subject';
+
+  @override
+  String get libraryShelfStarred => 'Starred';
+
+  @override
+  String get libraryShelfLastWeek => 'Last 7 days';
+
+  @override
+  String get libraryShelfLastMonth => 'Last month';
+
+  @override
+  String get libraryShelfOlder => 'Older';
 }
 
 /// The translations for English, as used in the United Kingdom (`en_GB`).

apps/learn/halkyon_learn_flutter/lib/src/core/device_settings/bloc/device_settings_bloc.dart

@@ -47,6 +47,9 @@ class DeviceSettingsBloc(
     on<DeviceSettingsEventCheckGrammarChosen>(
       (event, emit) => _apply(emit, state.withCheckGrammar(event.on)),
     );
+    on<DeviceSettingsEventLibrarySubjectChosen>(
+      (event, emit) => _apply(emit, state.withLibrarySubject(event.subject)),
+    );
     on<DeviceSettingsEventSidebarChosen>(
       (event, emit) => _apply(emit, state.withSidebar(open: event.open)),
     );
@@ -151,6 +154,14 @@ final class const DeviceSettingsEventCheckGrammarChosen(final bool on)
   List<Object?> get props => [on];
 }
 
+/// The person narrows the library to the subject [subject] (its ID), or
+/// shows all subjects again (null).
+final class const DeviceSettingsEventLibrarySubjectChosen(final String? subject)
+    extends DeviceSettingsEvent {
+  @override
+  List<Object?> get props => [subject];
+}
+
 /// The person opens the rail into the sidebar, or closes it ([open]).
 final class const DeviceSettingsEventSidebarChosen(final bool open)
     extends DeviceSettingsEvent {

apps/learn/halkyon_learn_flutter/lib/src/core/device_settings/data/device_settings_store.dart

@@ -78,6 +78,10 @@ class FileDeviceSettingsStore(final File _file) implements DeviceSettingsStore {
         },
         checkGrammar: json['checkGrammar'] != false,
         sidebarOpen: json['sidebarOpen'] == true,
+        librarySubject: switch (json['librarySubject']) {
+          final String id => id,
+          _ => null,
+        },
       );
     } on FormatException catch (error) {
       log('The device settings are unreadable', name: 'Settings', error: error);
@@ -107,6 +111,7 @@ class FileDeviceSettingsStore(final File _file) implements DeviceSettingsStore {
         },
         'checkGrammar': settings.checkGrammar,
         if (settings.sidebarOpen) 'sidebarOpen': true,
+        'librarySubject': ?settings.librarySubject,
       }),
       flush: true,
     );

apps/learn/halkyon_learn_flutter/lib/src/core/device_settings/model/device_settings.dart

@@ -67,6 +67,10 @@ final class const DeviceSettings({
   /// (decided 10 Nov 2026), halkyon_app's `Look.sidebarOpen` kept here.
   final bool sidebarOpen = false,
 
+  /// The ID of the subject the library is narrowed to (ADR 0041, D8,
+  /// decided 10 Nov 2026); null for all subjects.
+  final String? librarySubject,
+
   /// Whether the editors underline what the instance's LanguageTool finds
   /// as the person writes (ADR 0024, Settings › Writing): on unless turned
   /// off; nothing is checked where the instance checks nothing.
@@ -119,6 +123,10 @@ final class const DeviceSettings({
   /// These settings with the sidebar [open] or closed.
   DeviceSettings withSidebar({required bool open}) => _copy(sidebarOpen: open);
 
+  /// These settings with the library narrowed to the subject [id]; null
+  /// for all subjects.
+  DeviceSettings withLibrarySubject(String? id) => _copy(librarySubject: id);
+
   /// These settings with writing checked as it is written [on] or off.
   DeviceSettings withCheckGrammar(bool on) => _copy(checkGrammar: on);
 
@@ -136,6 +144,7 @@ final class const DeviceSettings({
     Map<String, LlmEffort>? askEfforts,
     bool? checkGrammar,
     bool? sidebarOpen,
+    Object? librarySubject = _keep,
   }) => DeviceSettings(
     appearance: appearance ?? this.appearance,
     language: identical(language, _keep) ? this.language : language as String?,
@@ -154,6 +163,9 @@ final class const DeviceSettings({
     askEfforts: askEfforts ?? this.askEfforts,
     checkGrammar: checkGrammar ?? this.checkGrammar,
     sidebarOpen: sidebarOpen ?? this.sidebarOpen,
+    librarySubject: identical(librarySubject, _keep)
+        ? this.librarySubject
+        : librarySubject as String?,
   );
 
   @override
@@ -171,6 +183,7 @@ final class const DeviceSettings({
     askEfforts,
     checkGrammar,
     sidebarOpen,
+    librarySubject,
   ];
 }
 

apps/learn/halkyon_learn_flutter/lib/src/feature/study/model/study_progress.dart

@@ -29,6 +29,10 @@ final class const StudyProgress({
 
   /// How many files (PDFs) there are with the cards, counted as notes are.
   final int files = 0,
+
+  /// When a card among them was last rated, in UTC; null when none was.
+  /// The library shelves by it (ADR 0041, D8).
+  final DateTime? lastStudied,
 }) with Equatable {
   /// The share of mature cards, from 0 to 1; null without cards.
   double? get mastery => cards == 0 ? null : mature / cards;
@@ -47,6 +51,7 @@ final class const StudyProgress({
     notes: notes,
     paths: paths,
     files: files,
+    lastStudied: lastStudied,
   );
 
   @override
@@ -58,6 +63,7 @@ final class const StudyProgress({
     notes,
     paths,
     files,
+    lastStudied,
   ];
 }
 
@@ -88,5 +94,12 @@ StudyProgress studyProgress({
     due: queue.length,
     reviewsDue: queue.where((id) => !(schedules[id]?.isNew ?? true)).length,
     mature: cards.where((id) => schedules[id]?.isMature ?? false).length,
+    lastStudied: cards.fold<DateTime?>(
+      null,
+      (last, id) => switch (schedules[id]?.lastReviewedAt) {
+        final at? when last == null || at.isAfter(last) => at,
+        _ => last,
+      },
+    ),
   );
 }

apps/learn/halkyon_learn_flutter/lib/src/feature/studyset/model/library_shelves.dart

@@ -0,0 +1,100 @@
+import 'package:halkyon_learn_flutter/src/core/sync/uuid_time.dart';
+import 'package:halkyon_learn_flutter/src/feature/study/model/study_progress.dart';
+import 'package:halkyon_learn_flutter/src/feature/studyset/model/studyset.dart';
+import 'package:halkyon_learn_flutter/src/feature/studyset/model/subject.dart';
+
+/// The shelves of the library (ADR 0041, D8, decided by the developer, 10
+/// Nov 2026, as Vaia's), in the order they show.
+enum LibraryShelf() {
+  /// The starred studysets, however long ago they were used.
+  starred,
+
+  /// Studied or changed in the last 7 days.
+  lastWeek,
+
+  /// Within the last 30 days, before those.
+  lastMonth,
+
+  /// Before.
+  older,
+}
+
+/// When [studyset] was last used: the later of when a card in it was last
+/// rated ([StudyProgress.lastStudied]) and when it last changed itself;
+/// for one from before changes were recorded, when it was made, as its
+/// UUID v7 says. Null where nothing says.
+DateTime? lastUsedOf(Studyset studyset, StudyProgress? progress) {
+  DateTime? latest;
+  for (final at in [
+    progress?.lastStudied,
+    studyset.updatedAt ?? uuidV7Time(studyset.id),
+  ]) {
+    if (at != null && (latest == null || at.isAfter(latest))) {
+      latest = at;
+    }
+  }
+  return latest;
+}
+
+/// [studysets] on their shelves at [now], each shelf's most recently used
+/// first; a shelf without any is left out.
+Map<LibraryShelf, List<Studyset>> shelvesOf(
+  List<Studyset> studysets, {
+  required Map<String, StudyProgress> progress,
+  required DateTime now,
+}) {
+  final used = {
+    for (final studyset in studysets)
+      studyset.id: lastUsedOf(studyset, progress[studyset.id]),
+  };
+  LibraryShelf shelfOf(Studyset studyset) {
+    if (studyset.isStarred) {
+      return LibraryShelf.starred;
+    }
+    final age = switch (used[studyset.id]) {
+      final at? => now.difference(at),
+      null => null,
+    };
+    return switch (age) {
+      final age? when age < const Duration(days: 7) => LibraryShelf.lastWeek,
+      final age? when age < const Duration(days: 30) => LibraryShelf.lastMonth,
+      _ => LibraryShelf.older,
+    };
+  }
+
+  final sorted = [...studysets]
+    ..sort(
+      (a, b) => switch ((used[a.id], used[b.id])) {
+        (final a?, final b?) when a != b => b.compareTo(a),
+        (_?, null) => -1,
+        (null, _?) => 1,
+        _ => b.id.compareTo(a.id),
+      },
+    );
+  final shelves = <LibraryShelf, List<Studyset>>{};
+  for (final studyset in sorted) {
+    (shelves[shelfOf(studyset)] ??= []).add(studyset);
+  }
+  return {
+    for (final shelf in LibraryShelf.values) shelf: ?shelves[shelf],
+  };
+}
+
+/// The subject [studyset] counts under in the library: its own, or
+/// [Subject.other] for none or one this app doesn't know.
+Subject librarySubjectOf(Studyset studyset) =>
+    Subject.byId(studyset.content.subject) ?? Subject.other;
+
+/// Whether [studyset] is listed when the library is narrowed to [subject].
+bool filedUnder(Studyset studyset, Subject subject) =>
+    librarySubjectOf(studyset) == subject;
+
+/// The subjects [studysets] are filed under, each once, in [Subject]'s
+/// order: what the library's dropdown offers.
+List<Subject> subjectsInUse(List<Studyset> studysets) {
+  final used = {for (final studyset in studysets) librarySubjectOf(studyset)};
+  return [
+    for (final subject in Subject.values)
+      if (used.contains(subject)) subject,
+  ];
+}

apps/learn/halkyon_learn_flutter/lib/src/feature/studyset/widget/library_screen.dart

@@ -1,11 +1,13 @@
 import 'dart:async';
 
+import 'package:clock/clock.dart';
 import 'package:flutter_bloc/flutter_bloc.dart';
 import 'package:go_router/go_router.dart';
 import 'package:halkyon_app/halkyon_app.dart';
 import 'package:halkyon_learn_flutter/l10n/app_localizations.dart';
 import 'package:halkyon_learn_flutter/src/core/dependencies/dependencies.dart';
 import 'package:halkyon_learn_flutter/src/core/dependencies/person_dependencies.dart';
+import 'package:halkyon_learn_flutter/src/core/device_settings/bloc/device_settings_bloc.dart';
 import 'package:halkyon_learn_flutter/src/core/router/route_paths.dart';
 import 'package:halkyon_learn_flutter/src/feature/anki/widget/anki_import_sheet.dart';
 import 'package:halkyon_learn_flutter/src/feature/archive/widget/import_archive.dart';
@@ -16,8 +18,10 @@ import 'package:halkyon_learn_flutter/src/feature/publish/widget/share_button.da
 import 'package:halkyon_learn_flutter/src/feature/study/model/study_progress.dart';
 import 'package:halkyon_learn_flutter/src/feature/study/widget/start_studying.dart';
 import 'package:halkyon_learn_flutter/src/feature/studyset/bloc/studyset_list_bloc.dart';
+import 'package:halkyon_learn_flutter/src/feature/studyset/model/library_shelves.dart';
 import 'package:halkyon_learn_flutter/src/feature/studyset/model/studyset.dart';
 import 'package:halkyon_learn_flutter/src/feature/studyset/model/studyset_tree.dart';
+import 'package:halkyon_learn_flutter/src/feature/studyset/model/subject.dart';
 import 'package:halkyon_learn_flutter/src/feature/studyset/widget/delete_studyset.dart';
 import 'package:halkyon_learn_flutter/src/feature/studyset/widget/deleted_studyset_card.dart';
 import 'package:halkyon_learn_flutter/src/feature/studyset/widget/studyset_actions.dart';
@@ -25,13 +29,16 @@ import 'package:halkyon_learn_flutter/src/feature/studyset/widget/studyset_card.
 import 'package:halkyon_learn_flutter/src/feature/studyset/widget/studyset_language_scope.dart';
 import 'package:halkyon_learn_flutter/src/feature/studyset/widget/studyset_navigation.dart';
 import 'package:halkyon_learn_flutter/src/feature/studyset/widget/studyset_sheet.dart';
+import 'package:halkyon_learn_flutter/src/feature/studyset/widget/subject_picker.dart';
 import 'package:halkyon_ui/halkyon_ui.dart';
 import 'package:lucide_icons_flutter/lucide_icons.dart';
 import 'package:material_ui/material_ui.dart';
 
-/// The library tab: the person's studysets as cards in a grid, starred ones
+/// The library tab: the person's studysets as cards in a grid, on shelves
-/// first, and archived ones in a section of their own below, closed until
+/// as Vaia's (ADR 0041, D8): the starred, then those studied or changed in
-/// opened. A search above the grid finds studysets by title and secondary
+/// the last 7 days, the last month and before, narrowed to one subject
+/// where the person chose one; archived ones in a section of their own
+/// below, closed until opened. A search above the grid finds studysets by title and secondary
 /// title, archived ones too.
 ///
 /// Creating a studyset is a floating button in compact windows, which slides
@@ -494,7 +501,7 @@ class _GridState() extends State<_Grid> {
   // a row lines up.
   static const _layout = SliverGridDelegateWithMaxCrossAxisExtent(
     maxCrossAxisExtent: 400,
-    mainAxisExtent: 188,
+    mainAxisExtent: 212,
     crossAxisSpacing: _spacing,
     mainAxisSpacing: _spacing,
   );
@@ -512,19 +519,34 @@ class _GridState() extends State<_Grid> {
     _published = context.watch<PublishedCubit>().state;
     final query = widget.query.trim().toLowerCase();
     final searching = query.isNotEmpty;
+    // Narrowed to the subject the device keeps, while one is in use (ADR
+    // 0041, D8); all of them otherwise.
+    final inUse = subjectsInUse(widget.studysets);
+    final kept = Subject.byId(
+      context.select<DeviceSettingsBloc, String?>(
+        (settings) => settings.state.librarySubject,
+      ),
+    );
+    final subject = inUse.contains(kept) ? kept : null;
+    bool listed(Studyset studyset) =>
+        (subject == null || filedUnder(studyset, subject)) &&
+        (!searching || _matches(studyset, query));
     final active = [
       for (final studyset in widget.studysets)
-        if (!studyset.isArchived && (!searching || _matches(studyset, query)))
+        if (!studyset.isArchived && listed(studyset)) studyset,
-          studyset,
     ];
+    final shelves = shelvesOf(
+      active,
+      progress: widget.progress,
+      now: clock.now().toUtc(),
+    );
     final archived = [
       for (final studyset in widget.studysets)
-        if (studyset.isArchived && (!searching || _matches(studyset, query)))
+        if (studyset.isArchived && listed(studyset)) studyset,
-          studyset,
     ];
     final deleted = [
       for (final studyset in widget.deleted)
-        if (!searching || _matches(studyset, query)) studyset,
+        if (listed(studyset)) studyset,
     ];
     // A search opens the sections, so their matches show; closing the
     // search brings them back as the person left them.
@@ -549,14 +571,26 @@ class _GridState() extends State<_Grid> {
             SliverToBoxAdapter(
               child: _PrivateOffer(studysets: widget.studysets),
             ),
-          SliverPadding(
+          SliverToBoxAdapter(
-            padding: const EdgeInsets.all(_spacing),
+            child: _SubjectDropdown(subjects: inUse, selected: subject),
-            sliver: SliverGrid.builder(
-              gridDelegate: _layout,
-              itemCount: active.length,
-              itemBuilder: (context, index) => _card(context, active[index]),
-            ),
           ),
+          for (final MapEntry(key: shelf, value: listed)
+              in shelves.entries) ...[
+            SliverToBoxAdapter(child: _ShelfHeading(shelf)),
+            SliverPadding(
+              padding: const EdgeInsets.fromLTRB(
+                _spacing,
+                0,
+                _spacing,
+                _spacing,
+              ),
+              sliver: SliverGrid.builder(
+                gridDelegate: _layout,
+                itemCount: listed.length,
+                itemBuilder: (context, index) => _card(context, listed[index]),
+              ),
+            ),
+          ],
           if (archived.isNotEmpty) ...[
             SliverToBoxAdapter(
               child: _SectionHeader(
@@ -882,6 +916,93 @@ class const _DeletedNote() extends StatelessWidget {
   }
 }
 
+/// The subject the library is narrowed to, chosen among those in use (ADR
+/// 0041, D8): all at first, the choice kept on the device.
+class const _SubjectDropdown({
+  required final List<Subject> subjects,
+  required final Subject? selected,
+}) extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+    final l10n = AppLocalizations.of(context);
+    final theme = Theme.of(context);
+    void choose(Subject? subject) => context.read<DeviceSettingsBloc>().add(
+      DeviceSettingsEventLibrarySubjectChosen(subject?.id),
+    );
+    return Padding(
+      padding: const EdgeInsets.fromLTRB(_GridState._spacing, 4, 16, 0),
+      child: Align(
+        alignment: AlignmentDirectional.centerStart,
+        child: MenuAnchor(
+          menuChildren: [
+            MenuItemButton(
+              leadingIcon: const Icon(LucideIcons.layoutGrid, size: 20),
+              trailingIcon: selected == null
+                  ? const Icon(LucideIcons.check, size: 18)
+                  : null,
+              onPressed: () => choose(null),
+              child: Text(l10n.libraryAllSubjects),
+            ),
+            for (final subject in subjects)
+              MenuItemButton(
+                leadingIcon: SubjectBadge(subject, size: 24),
+                trailingIcon: subject == selected
+                    ? const Icon(LucideIcons.check, size: 18)
+                    : null,
+                onPressed: () => choose(subject),
+                child: Text(subject.nameIn(l10n)),
+              ),
+          ],
+          builder: (context, menu, _) => Tooltip(
+            message: l10n.librarySubjectTooltip,
+            child: TextButton.icon(
+              onPressed: () => menu.isOpen ? menu.close() : menu.open(),
+              icon: switch (selected) {
+                final subject? => SubjectBadge(subject, size: 22),
+                null => const Icon(LucideIcons.layoutGrid),
+              },
+              label: Row(
+                mainAxisSize: MainAxisSize.min,
+                children: [
+                  Text(
+                    selected?.nameIn(l10n) ?? l10n.libraryAllSubjects,
+                    style: theme.textTheme.titleSmall,
+                  ),
+                  const SizedBox(width: 4),
+                  const Icon(LucideIcons.chevronDown, size: 18),
+                ],
+              ),
+            ),
+          ),
+        ),
+      ),
+    );
+  }
+}
+
+/// The heading over a shelf of the library (ADR 0041, D8).
+class const _ShelfHeading(final LibraryShelf shelf) extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+    final l10n = AppLocalizations.of(context);
+    final theme = Theme.of(context);
+    return Padding(
+      padding: const EdgeInsets.fromLTRB(16, 12, 16, 8),
+      child: Text(
+        switch (shelf) {
+          LibraryShelf.starred => l10n.libraryShelfStarred,
+          LibraryShelf.lastWeek => l10n.libraryShelfLastWeek,
+          LibraryShelf.lastMonth => l10n.libraryShelfLastMonth,
+          LibraryShelf.older => l10n.libraryShelfOlder,
+        },
+        style: theme.textTheme.titleSmall?.copyWith(
+          color: theme.colorScheme.onSurfaceVariant,
+        ),
+      ),
+    );
+  }
+}
+
 /// A section's header (the archive, Recently deleted): it opens and closes
 /// the section, and counts what rests there.
 class const _SectionHeader({

apps/learn/halkyon_learn_flutter/lib/src/feature/studyset/widget/studyset_card.dart

@@ -1,17 +1,20 @@
 import 'package:flutter/services.dart';
 import 'package:halkyon_learn_flutter/l10n/app_localizations.dart';
+import 'package:halkyon_learn_flutter/src/core/l10n/content_languages.dart';
 import 'package:halkyon_learn_flutter/src/core/theme/learn_colors.dart';
 import 'package:halkyon_learn_flutter/src/core/widget/content_counts_line.dart';
 import 'package:halkyon_learn_flutter/src/feature/publish/widget/share_button.dart';
 import 'package:halkyon_learn_flutter/src/feature/study/model/study_progress.dart';
 import 'package:halkyon_learn_flutter/src/feature/studyset/model/studyset.dart';
+import 'package:halkyon_learn_flutter/src/feature/studyset/model/subject.dart';
 import 'package:halkyon_learn_flutter/src/feature/studyset/widget/studyset_actions.dart';
 import 'package:halkyon_ui/halkyon_ui.dart';
 import 'package:lucide_icons_flutter/lucide_icons.dart';
 import 'package:material_ui/material_ui.dart';
 
 /// A studyset in the library. On top a bar in its colour; below, its title
-/// with the secondary title as a kicker, and what it holds; beside the title
+/// with the secondary title as a kicker, the subject it is filed under
+/// (ADR 0041, D8), and what it holds; beside the title
 /// the action it invites, inside a ring its mature cards fill, with how many
 /// cards are due; at the bottom who made it, its star, whether it is
 /// public and its options.
@@ -124,6 +127,14 @@ class _StudysetCardState() extends State<StudysetCard> {
                                   onAdd: widget.onOpen,
                                 ),
                         ),
+                        if (Subject.byId(studyset.content.subject)
+                            case final subject?) ...[
+                          const SizedBox(height: 2),
+                          _SubjectLine(
+                            subject: subject,
+                            language: studyset.content.frontLanguage,
+                          ),
+                        ],
                         const SizedBox(height: 8),
                         _Holds(progress: widget.progress),
                       ],
@@ -164,6 +175,41 @@ String _by(AppLocalizations l10n, Studyset studyset) {
   return handle == null ? l10n.studysetByOthers : l10n.studysetByAuthor(handle);
 }
 
+/// The subject a studyset is filed under, with its icon; a language course
+/// names its language beside (ADR 0041, D3).
+class const _SubjectLine({
+  required final Subject subject,
+  required final String? language,
+}) extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+    final theme = Theme.of(context);
+    final l10n = AppLocalizations.of(context);
+    final style = theme.textTheme.bodySmall?.copyWith(
+      color: theme.colorScheme.onSurfaceVariant,
+    );
+    final name = subject.nameIn(l10n);
+    return Row(
+      children: [
+        Icon(subject.icon, size: 14, color: style?.color),
+        const SizedBox(width: 6),
+        Expanded(
+          child: Text(
+            switch (language) {
+              final tag? when subject == Subject.languages =>
+                '$name · ${contentLanguageName(l10n, tag)}',
+              _ => name,
+            },
+            style: style,
+            maxLines: 1,
+            overflow: TextOverflow.ellipsis,
+          ),
+        ),
+      ],
+    );
+  }
+}
+
 /// What the studyset holds; a muted bar of the same height while that is
 /// still being worked out, which "No content yet" would misstate.
 class const _Holds({required final StudyProgress? progress})

apps/learn/halkyon_learn_flutter/test/core/device_settings/device_settings_store_test.dart

@@ -65,6 +65,18 @@ void main() {
     expect(read.withLanguage('de').sidebarOpen, isTrue);
   });
 
+  test('keeps the subject the library is narrowed to (ADR 0041, D8), all '
+      'until one is chosen', () async {
+    expect((await store.read()).librarySubject, isNull);
+
+    await store.write(const DeviceSettings(librarySubject: 'languages'));
+
+    final read = await FileDeviceSettingsStore(file).read();
+    expect(read.librarySubject, 'languages');
+    expect(read.withLibrarySubject(null).librarySubject, isNull);
+    expect(read.withLanguage('de').librarySubject, 'languages');
+  });
+
   test('shows the Pomodoro timer while studying until told not to', () async {
     expect((await store.read()).pomodoroWhileStudying, isTrue);
 

apps/learn/halkyon_learn_flutter/test/feature/study/model/study_progress_test.dart

@@ -39,6 +39,46 @@ void main() {
     expect(progress.mastery, 0.25);
   });
 
+  test('names when a card among them was last rated, for the library to '
+      "shelve by (the developer's decision of 10 Nov 2026)", () {
+    CardSchedule ratedAt(DateTime at) => CardSchedule(
+      reviewCount: 1,
+      due: now.add(const Duration(days: 2)),
+      firstReviewedAt: at,
+      lastReviewedAt: at,
+    );
+    final progress = studyProgress(
+      groups: [
+        ['old', 'new'],
+        ['recent'],
+      ],
+      schedules: {
+        'old': ratedAt(DateTime.utc(2026, 9)),
+        'new': fresh,
+        'recent': ratedAt(DateTime.utc(2026, 10, 4)),
+      },
+      now: now,
+      startOfToday: startOfToday,
+    );
+
+    expect(progress.lastStudied, DateTime.utc(2026, 10, 4));
+    expect(
+      progress.withContent(notes: 1, paths: 0, files: 0).lastStudied,
+      DateTime.utc(2026, 10, 4),
+    );
+    expect(
+      studyProgress(
+        groups: [
+          ['new'],
+        ],
+        schedules: {'new': fresh},
+        now: now,
+        startOfToday: startOfToday,
+      ).lastStudied,
+      isNull,
+    );
+  });
+
   test('counts only as many new cards as today still starts', () {
     final order = [for (var i = 0; i < 30; i++) 'n$i'];
 

apps/learn/halkyon_learn_flutter/test/feature/studyset/model/library_shelves_test.dart

@@ -0,0 +1,125 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:halkyon_editor/halkyon_editor.dart';
+import 'package:halkyon_learn_flutter/src/feature/study/model/study_progress.dart';
+import 'package:halkyon_learn_flutter/src/feature/studyset/model/library_shelves.dart';
+import 'package:halkyon_learn_flutter/src/feature/studyset/model/studyset.dart';
+import 'package:halkyon_learn_flutter/src/feature/studyset/model/studyset_content.dart';
+import 'package:halkyon_learn_flutter/src/feature/studyset/model/subject.dart';
+
+/// The library as Vaia's (ADR 0041, D8, the developer's decisions of 10 Nov
+/// 2026): the starred on top, then by when each was last studied or changed,
+/// in the last 7 days, the last month and before; narrowed to a subject.
+void main() {
+  final now = DateTime.utc(2026, 11, 10, 12);
+
+  Studyset studyset(
+    String id, {
+    DateTime? updatedAt,
+    bool starred = false,
+    String? subject,
+  }) => Studyset(
+    id: id,
+    content: StudysetContent(
+      title: id,
+      description: RichContent.empty(),
+      subject: subject,
+    ),
+    updatedAt: updatedAt,
+    starredAt: starred ? DateTime.utc(2026) : null,
+  );
+
+  StudyProgress studied(DateTime at) => StudyProgress(
+    cards: 1,
+    due: 0,
+    reviewsDue: 0,
+    mature: 0,
+    lastStudied: at,
+  );
+
+  test('shelves by the later of the last rating and the last change, the '
+      'most recent first in each shelf', () {
+    final shelves = shelvesOf(
+      [
+        studyset('changed-today', updatedAt: now),
+        studyset('studied-3-days-ago', updatedAt: DateTime.utc(2026, 1, 2)),
+        studyset('changed-2-weeks-ago', updatedAt: DateTime.utc(2026, 10, 27)),
+        studyset('old', updatedAt: DateTime.utc(2026, 1, 2)),
+        studyset('yesterday', updatedAt: now.subtract(const Duration(days: 1))),
+      ],
+      progress: {
+        'studied-3-days-ago': studied(now.subtract(const Duration(days: 3))),
+        'old': studied(DateTime.utc(2026, 2)),
+      },
+      now: now,
+    );
+
+    List<String> ids(LibraryShelf shelf) => [
+      for (final studyset in shelves[shelf] ?? const <Studyset>[]) studyset.id,
+    ];
+    expect(ids(LibraryShelf.lastWeek), [
+      'changed-today',
+      'yesterday',
+      'studied-3-days-ago',
+    ]);
+    expect(ids(LibraryShelf.lastMonth), ['changed-2-weeks-ago']);
+    expect(ids(LibraryShelf.older), ['old']);
+    expect(shelves.keys, isNot(contains(LibraryShelf.starred)));
+  });
+
+  test('the starred stand on their own shelf on top, however long ago', () {
+    final shelves = shelvesOf(
+      [
+        studyset('recent', updatedAt: now),
+        studyset('starred-old', updatedAt: DateTime.utc(2025), starred: true),
+      ],
+      progress: const {},
+      now: now,
+    );
+
+    expect(shelves.keys.first, LibraryShelf.starred);
+    expect(shelves[LibraryShelf.starred]!.single.id, 'starred-old');
+    expect(shelves[LibraryShelf.lastWeek]!.single.id, 'recent');
+  });
+
+  test('a studyset from before changes were recorded counts from when it '
+      'was made, its UUID v7 saying when', () {
+    final shelves = shelvesOf(
+      [
+        // Made at 2026-11-08 00:00 UTC.
+        studyset('01a1b874-6c00-7000-8000-000000000000'),
+        studyset('not-a-uuid'),
+      ],
+      progress: const {},
+      now: now,
+    );
+
+    expect(shelves[LibraryShelf.lastWeek]!.single.id, startsWith('01a1'));
+    expect(shelves[LibraryShelf.older]!.single.id, 'not-a-uuid');
+  });
+
+  test("the subjects in use, in the list's order; one without a subject, "
+      'or one this app does not know, counts as Other', () {
+    final studysets = [
+      studyset('rust', subject: 'software-engineering'),
+      studyset('japanese', subject: 'languages'),
+      studyset('notes'),
+      studyset('origami', subject: 'paper-folding'),
+      studyset('korean', subject: 'languages'),
+    ];
+
+    expect(subjectsInUse(studysets), [
+      Subject.softwareEngineering,
+      Subject.languages,
+      Subject.other,
+    ]);
+    expect(
+      [
+        for (final studyset in studysets)
+          if (filedUnder(studyset, Subject.other)) studyset.id,
+      ],
+      ['notes', 'origami'],
+    );
+    expect(filedUnder(studysets.first, Subject.softwareEngineering), isTrue);
+    expect(filedUnder(studysets.first, Subject.languages), isFalse);
+  });
+}

apps/learn/halkyon_learn_flutter/test/feature/studyset/widget/library_screen_test.dart

@@ -5,6 +5,7 @@ import 'package:flutter_test/flutter_test.dart';
 import 'package:halkyon_app/testing.dart';
 import 'package:halkyon_editor/halkyon_editor.dart';
 import 'package:halkyon_learn_flutter/src/core/content/published_ref.dart';
+import 'package:halkyon_learn_flutter/src/core/device_settings/model/device_settings.dart';
 import 'package:halkyon_learn_flutter/src/core/router/route_paths.dart';
 import 'package:halkyon_learn_flutter/src/feature/card/data/card_repository.dart';
 import 'package:halkyon_learn_flutter/src/feature/card/model/card_content.dart';
@@ -22,6 +23,7 @@ import 'package:material_ui/material_ui.dart';
 import '../../../support/fake_card_gateway.dart';
 import '../../../support/fake_publish_gateway.dart';
 import '../../../support/fake_studyset_gateway.dart';
+import '../../../support/memory_device_settings_store.dart';
 import '../../../support/memory_published_store.dart';
 import '../../../support/pump_app.dart';
 import '../../../support/test_dependencies.dart';
@@ -76,6 +78,7 @@ void main() {
     DateTime? starredAt,
     String? publishedAt,
     String? subject,
+    DateTime? updatedAt,
     bool private = false,
   }) => gateway.receiveRemoteChange(
     id,
@@ -93,6 +96,7 @@ void main() {
     ),
     archivedAt: archivedAt,
     starredAt: starredAt,
+    updatedAt: updatedAt,
     isPrivate: private,
   );
 
@@ -373,6 +377,133 @@ void main() {
     });
   });
 
+  group("shelved and narrowed as Vaia's library (ADR 0041, D8, the "
+      "developer's decisions of 10 Nov 2026)", () {
+    final now = DateTime.now().toUtc();
+    DateTime daysAgo(int days) => now.subtract(Duration(days: days));
+
+    testWidgets('the starred on top, then the last 7 days, the last month '
+        'and before, each under its heading; the archive last', (
+      tester,
+    ) async {
+      stored('old', 'Old', updatedAt: daysAgo(90));
+      stored('recent', 'Recent', updatedAt: daysAgo(1));
+      stored('weeks', 'Weeks', updatedAt: daysAgo(14));
+      stored('star', 'Star', updatedAt: daysAgo(200), starredAt: daysAgo(1));
+      stored('rest', 'Rest', updatedAt: daysAgo(2), archivedAt: daysAgo(2));
+      await showLibrary(tester);
+      double topOf(Finder finder) => tester.getTopLeft(finder).dy;
+
+      final order = [
+        'Starred',
+        'Star',
+        'Last 7 days',
+        'Recent',
+        'Last month',
+        'Weeks',
+        'Older',
+        'Old',
+        'Archived',
+      ];
+      for (final (before, after) in [
+        for (var i = 1; i < order.length; i++) (order[i - 1], order[i]),
+      ]) {
+        expect(
+          topOf(find.text(before).first),
+          lessThan(topOf(find.text(after).first)),
+          reason: '$before above $after',
+        );
+      }
+    });
+
+    testWidgets('the dropdown offers the subjects in use, narrows the '
+        'library to one, and the device keeps it', (tester) async {
+      final settings = MemoryDeviceSettingsStore();
+      stored('rust', 'Rust', subject: 'software-engineering');
+      stored('japanese', 'Japanese', subject: 'languages');
+      stored('notes', 'Notes');
+      tester.view.physicalSize = const Size(1200, 1600);
+      tester.view.devicePixelRatio = 1;
+      addTearDown(tester.view.reset);
+      await pumpApp(
+        tester,
+        testDependencies(
+          studysetGateway: gateway,
+          cardGateway: cardGateway,
+          deviceSettingsStore: settings,
+        ),
+        location: RoutePaths.library,
+      );
+
+      await tester.tap(find.text('All subjects'));
+      await tester.pumpAndSettle();
+      expect(
+        find.widgetWithText(MenuItemButton, 'Software engineering'),
+        findsOneWidget,
+      );
+      expect(
+        find.widgetWithText(MenuItemButton, 'Other / not sure'),
+        findsOneWidget,
+      );
+      expect(find.text('Biology'), findsNothing, reason: 'not in use');
+      await tester.tap(find.widgetWithText(MenuItemButton, 'Languages'));
+      await tester.pumpAndSettle();
+
+      expect(find.widgetWithText(StudysetCard, 'Japanese'), findsOneWidget);
+      expect(find.widgetWithText(StudysetCard, 'Rust'), findsNothing);
+      expect(find.widgetWithText(StudysetCard, 'Notes'), findsNothing);
+      expect(settings.writes.last.librarySubject, 'languages');
+
+      await tester.tap(find.byTooltip('Show one subject'));
+      await tester.pumpAndSettle();
+      await tester.tap(find.widgetWithText(MenuItemButton, 'Other / not sure'));
+      await tester.pumpAndSettle();
+      expect(find.widgetWithText(StudysetCard, 'Notes'), findsOneWidget);
+      expect(find.widgetWithText(StudysetCard, 'Japanese'), findsNothing);
+    });
+
+    testWidgets('opens on the subject kept', (
+      tester,
+    ) async {
+      stored('rust', 'Rust', subject: 'software-engineering');
+      stored('japanese', 'Japanese', subject: 'languages');
+      tester.view.physicalSize = const Size(1200, 1600);
+      tester.view.devicePixelRatio = 1;
+      addTearDown(tester.view.reset);
+      await pumpApp(
+        tester,
+        testDependencies(
+          studysetGateway: gateway,
+          cardGateway: cardGateway,
+          deviceSettings: const DeviceSettings(librarySubject: 'languages'),
+        ),
+        location: RoutePaths.library,
+      );
+      expect(find.widgetWithText(StudysetCard, 'Rust'), findsNothing);
+      expect(find.widgetWithText(StudysetCard, 'Japanese'), findsOneWidget);
+    });
+
+    testWidgets('a subject kept but no longer in use shows all', (
+      tester,
+    ) async {
+      stored('rust', 'Rust', subject: 'software-engineering');
+      tester.view.physicalSize = const Size(1200, 1600);
+      tester.view.devicePixelRatio = 1;
+      addTearDown(tester.view.reset);
+      await pumpApp(
+        tester,
+        testDependencies(
+          studysetGateway: gateway,
+          cardGateway: cardGateway,
+          deviceSettings: const DeviceSettings(librarySubject: 'biology'),
+        ),
+        location: RoutePaths.library,
+      );
+      expect(find.widgetWithText(StudysetCard, 'Rust'), findsOneWidget);
+      expect(find.text('All subjects'), findsOneWidget);
+    });
+  });
+
   testWidgets('creating waits for a title', (tester) async {
     await showLibrary(tester);
     await tester.tap(find.text('Create studyset'));
@@ -482,9 +613,10 @@ void main() {
 
     await showLibrary(tester);
 
+    // On a shelf of their own above (ADR 0041, D8).
     expect(
-      tester.getTopLeft(find.text('German')).dx,
+      tester.getTopLeft(find.text('German')).dy,
-      lessThan(tester.getTopLeft(find.text('Japanese')).dx),
+      lessThan(tester.getTopLeft(find.text('Japanese')).dy),
     );
   });
 

apps/learn/halkyon_learn_flutter/test/feature/studyset/widget/studyset_card_test.dart

@@ -7,6 +7,7 @@ import 'package:halkyon_learn_flutter/src/core/theme/learn_theme.dart';
 import 'package:halkyon_learn_flutter/src/feature/study/model/study_progress.dart';
 import 'package:halkyon_learn_flutter/src/feature/studyset/model/studyset.dart';
 import 'package:halkyon_learn_flutter/src/feature/studyset/model/studyset_content.dart';
+import 'package:halkyon_learn_flutter/src/feature/studyset/model/subject.dart';
 import 'package:halkyon_learn_flutter/src/feature/studyset/widget/studyset_card.dart';
 import 'package:material_ui/material_ui.dart';
 
@@ -42,7 +43,7 @@ void main() {
           body: Center(
             child: SizedBox(
               width: 360,
-              height: 188,
+              height: 212,
               child: StudysetCard(
                 studyset: studyset ?? vocabulary,
                 progress: progress,
@@ -77,6 +78,47 @@ void main() {
     expect(find.text('52 flashcards'), findsOneWidget);
   });
 
+  group("names its subject under the title (ADR 0041, D8, the developer's "
+      'decisions of 10 Nov 2026)', () {
+    Studyset filed(String? subject, {String? front}) => Studyset(
+      id: 'set',
+      content: StudysetContent(
+        title: 'Japanese',
+        description: RichContent.empty(),
+        subject: subject,
+        frontLanguage: front,
+      ),
+    );
+
+    testWidgets('with its icon', (tester) async {
+      await show(tester, studyset: filed('software-engineering'));
+
+      final name = find.text('Software engineering');
+      expect(name, findsOneWidget);
+      expect(
+        tester.getTopLeft(name).dy,
+        greaterThan(tester.getBottomLeft(find.text('Japanese')).dy - 1),
+      );
+      expect(find.byIcon(Subject.softwareEngineering.icon), findsOneWidget);
+    });
+
+    testWidgets('a language course with its language', (tester) async {
+      await show(tester, studyset: filed('languages', front: 'ja'));
+
+      expect(find.text('Languages · Japanese'), findsOneWidget);
+    });
+
+    testWidgets('none, nor one this app does not know: nothing', (
+      tester,
+    ) async {
+      await show(tester, studyset: filed(null));
+      expect(find.byIcon(Subject.other.icon), findsNothing);
+
+      await show(tester, studyset: filed('paper-folding'));
+      expect(find.byIcon(Subject.other.icon), findsNothing);
+    });
+  });
+
   testWidgets('says when a studyset holds nothing yet', (tester) async {
     await show(
       tester,

docs/adr/0041-learn-subjects.md

@@ -1,8 +1,8 @@
 # ADR 0041: Learn's subjects: one per studyset, from a fixed list
 
 - **Status:** decided 9 and 10 Nov 2026 by the developer: D1 to D4 and D6 are their own
-  choices, D5 and D7 to D9 the agent's recommendations, which they confirmed. D1 to D7
+  choices, D5 and D7 to D9 and D8's details the agent's recommendations, which they
-  are built; D8 (the library) is next, and D10 later.
+  confirmed. D1 to D9 are built; D10 comes later.
 - **Builds on:** ADR 0002 (payloads and their schemas), ADR 0005 (publishing), ADR 0031
   (the palette's twenty colours).
 
@@ -33,7 +33,17 @@ narrowed to one.
   kept as it is and reads as no subject until another is picked.
 - **D8.** The library, as Vaia's: a dropdown of the subjects in use, cards naming the
   subject under the title with a stripe in its colour, sections by recency (the last 7
-  days, the last month, older), the archive last.
+  days, the last month, older), the archive last. Decided on 10 Nov 2026:
+  - recent by the later of a card's last rating in the studyset (or inside it) and the
+    studyset's own last change; one from before changes were recorded counts from when
+    it was made, as its UUID v7 says (`lastUsedOf`, `shelvesOf`);
+  - the starred on a shelf of their own on top, however long ago they were used;
+  - the subject chosen kept on the device (`DeviceSettings.librarySubject`), all
+    subjects where none is chosen or the one kept is no longer in use; a studyset
+    without a subject, or with one this version doesn't know, is under *Other*;
+  - the stripe is the studyset's own colour, the card's bar on top, which the subject
+    sets when picked; the subject's line shows the subject's icon, and a language course
+    its language beside (*Languages · Japanese*).
 - **D9.** Existing studysets: the agent writes a subject into each of the developer's 72
   archives not yet imported; those already in the library get one by hand.
 - **D10.** Explore filters by subject later; its releases don't keep the subject in a