feat(todo): a project's editor as Todoist's Add project, with sub-projects and favourites

Adding or editing a project was a dialog of a name and a colour. It is
now a screen of its own on a phone (a dialog where sheets are dialogs),
as Todoist's Add project: the name counted to 120, a description, the
colour, the parent project, a Favourite switch and the layout, Display's
pictures. All of it lives in the project's sealed payload, the schema
still 1, no server model or migration; the payload now keeps fields it
doesn't know, as a task's does.

A new project starts with the layout picked, and Display's Reset all
returns to that default rather than to List. Browse lists sub-projects
indented and folding under their parents, and the favourites in a group
above; a project is never offered as its own ancestor, and a parent's
deletion takes its sub-projects, naming them first, as Todoist's. ADR
0033, D28.

Whats-New: Add project and Edit project are a screen of their own, with a description, a parent project, Favourite and the layout.
Whats-New: Browse lists sub-projects under their parents and your favourite projects at the top.
Whats-New: Reset all in Display returns to the layout the project was made with.
Whats-New-De: „Projekt hinzufügen“ und „Projekt bearbeiten“ sind eine eigene Seite, mit Beschreibung, übergeordnetem Projekt, Favorit und Layout.
Whats-New-De: „Übersicht“ zeigt Unterprojekte unter ihren Projekten und deine Favoriten oben.
Whats-New-De: „Alles zurücksetzen“ in der Anzeige kehrt zum Layout zurück, mit dem das Projekt angelegt wurde.

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/todo/halkyon_todo_flutter/lib/l10n/app_de.arb

@@ -51,11 +51,18 @@
   "priority4": "Keine Priorität",
   "projectsTitle": "Projekte",
   "projectsNone": "Noch keine eigenen Projekte.",
-  "projectAdd": "Neues Projekt",
+  "projectAdd": "Projekt hinzufügen",
   "projectEdit": "Projekt bearbeiten",
   "projectDelete": "Projekt löschen",
   "projectDeleteTitle": "Projekt löschen?",
   "projectDeleteMessage": "„{name}“ wird mit seinen Abschnitten und Aufgaben auf all deinen Geräten gelöscht.",
+  "projectParent": "Übergeordnetes Projekt",
+  "projectNoParent": "Kein übergeordnetes Projekt",
+  "projectFavorite": "Favorit",
+  "favoritesTitle": "Favoriten",
+  "projectSubProjectsHide": "Unterprojekte ausblenden",
+  "projectSubProjectsShow": "Unterprojekte einblenden",
+  "projectDeleteWithSubProjects": "„{name}“ wird mit seinen Unterprojekten {subProjects} und ihren Abschnitten und Aufgaben auf all deinen Geräten gelöscht.",
   "projectEmptyTitle": "Hier ist nichts zu tun",
   "projectEmptyHint": "Füge mit dem Plus eine Aufgabe hinzu.",
   "projectGone": "Dieses Projekt wurde gelöscht.",

apps/todo/halkyon_todo_flutter/lib/l10n/app_en.arb

@@ -106,7 +106,7 @@
   "priority4": "No priority",
   "projectsTitle": "Projects",
   "projectsNone": "No projects of your own yet.",
-  "projectAdd": "New project",
+  "projectAdd": "Add project",
   "projectEdit": "Edit project",
   "projectDelete": "Delete project",
   "projectDeleteTitle": "Delete the project?",
@@ -118,6 +118,27 @@
       }
     }
   },
+  "projectParent": "Parent project",
+  "@projectParent": {
+    "description": "A project's editor (ADR 0033, D28), as Todoist's Add project."
+  },
+  "projectNoParent": "No parent",
+  "projectFavorite": "Favourite",
+  "favoritesTitle": "Favourites",
+  "projectSubProjectsHide": "Hide sub-projects",
+  "projectSubProjectsShow": "Show sub-projects",
+  "projectDeleteWithSubProjects": "\"{name}\" is deleted with its sub-projects {subProjects}, and their sections and tasks, on all your devices.",
+  "@projectDeleteWithSubProjects": {
+    "description": "Deleting a project with sub-projects (ADR 0033, D28): subProjects their names, separated by commas.",
+    "placeholders": {
+      "name": {
+        "type": "String"
+      },
+      "subProjects": {
+        "type": "String"
+      }
+    }
+  },
   "projectEmptyTitle": "Nothing to do here",
   "projectEmptyHint": "Add a task with the + button.",
   "projectGone": "This project was deleted.",

apps/todo/halkyon_todo_flutter/lib/l10n/app_en_US.arb

@@ -1,3 +1,5 @@
 {
-  "@@locale": "en_US"
+  "@@locale": "en_US",
+  "projectFavorite": "Favorite",
+  "favoritesTitle": "Favorites"
 }

apps/todo/halkyon_todo_flutter/lib/l10n/app_localizations.dart

@@ -409,7 +409,7 @@ abstract class AppLocalizations {
   /// No description provided for @projectAdd.
   ///
   /// In en, this message translates to:
-  /// **'New project'**
+  /// **'Add project'**
   String get projectAdd;
 
   /// No description provided for @projectEdit.
@@ -436,6 +436,48 @@ abstract class AppLocalizations {
   /// **'\"{name}\" is deleted with its sections and tasks, on all your devices.'**
   String projectDeleteMessage(String name);
 
+  /// A project's editor (ADR 0033, D28), as Todoist's Add project.
+  ///
+  /// In en, this message translates to:
+  /// **'Parent project'**
+  String get projectParent;
+
+  /// No description provided for @projectNoParent.
+  ///
+  /// In en, this message translates to:
+  /// **'No parent'**
+  String get projectNoParent;
+
+  /// No description provided for @projectFavorite.
+  ///
+  /// In en, this message translates to:
+  /// **'Favourite'**
+  String get projectFavorite;
+
+  /// No description provided for @favoritesTitle.
+  ///
+  /// In en, this message translates to:
+  /// **'Favourites'**
+  String get favoritesTitle;
+
+  /// No description provided for @projectSubProjectsHide.
+  ///
+  /// In en, this message translates to:
+  /// **'Hide sub-projects'**
+  String get projectSubProjectsHide;
+
+  /// No description provided for @projectSubProjectsShow.
+  ///
+  /// In en, this message translates to:
+  /// **'Show sub-projects'**
+  String get projectSubProjectsShow;
+
+  /// Deleting a project with sub-projects (ADR 0033, D28): subProjects their names, separated by commas.
+  ///
+  /// In en, this message translates to:
+  /// **'\"{name}\" is deleted with its sub-projects {subProjects}, and their sections and tasks, on all your devices.'**
+  String projectDeleteWithSubProjects(String name, String subProjects);
+
   /// No description provided for @projectEmptyTitle.
   ///
   /// In en, this message translates to:

apps/todo/halkyon_todo_flutter/lib/l10n/app_localizations_de.dart

@@ -191,7 +191,7 @@ class AppLocalizationsDe extends AppLocalizations {
   String get projectsNone => 'Noch keine eigenen Projekte.';
 
   @override
-  String get projectAdd => 'Neues Projekt';
+  String get projectAdd => 'Projekt hinzufügen';
 
   @override
   String get projectEdit => 'Projekt bearbeiten';
@@ -207,6 +207,29 @@ class AppLocalizationsDe extends AppLocalizations {
     return '„$name“ wird mit seinen Abschnitten und Aufgaben auf all deinen Geräten gelöscht.';
   }
 
+  @override
+  String get projectParent => 'Übergeordnetes Projekt';
+
+  @override
+  String get projectNoParent => 'Kein übergeordnetes Projekt';
+
+  @override
+  String get projectFavorite => 'Favorit';
+
+  @override
+  String get favoritesTitle => 'Favoriten';
+
+  @override
+  String get projectSubProjectsHide => 'Unterprojekte ausblenden';
+
+  @override
+  String get projectSubProjectsShow => 'Unterprojekte einblenden';
+
+  @override
+  String projectDeleteWithSubProjects(String name, String subProjects) {
+    return '„$name“ wird mit seinen Unterprojekten $subProjects und ihren Abschnitten und Aufgaben auf all deinen Geräten gelöscht.';
+  }
+
   @override
   String get projectEmptyTitle => 'Hier ist nichts zu tun';
 

apps/todo/halkyon_todo_flutter/lib/l10n/app_localizations_en.dart

@@ -191,7 +191,7 @@ class AppLocalizationsEn extends AppLocalizations {
   String get projectsNone => 'No projects of your own yet.';
 
   @override
-  String get projectAdd => 'New project';
+  String get projectAdd => 'Add project';
 
   @override
   String get projectEdit => 'Edit project';
@@ -207,6 +207,29 @@ class AppLocalizationsEn extends AppLocalizations {
     return '\"$name\" is deleted with its sections and tasks, on all your devices.';
   }
 
+  @override
+  String get projectParent => 'Parent project';
+
+  @override
+  String get projectNoParent => 'No parent';
+
+  @override
+  String get projectFavorite => 'Favourite';
+
+  @override
+  String get favoritesTitle => 'Favourites';
+
+  @override
+  String get projectSubProjectsHide => 'Hide sub-projects';
+
+  @override
+  String get projectSubProjectsShow => 'Show sub-projects';
+
+  @override
+  String projectDeleteWithSubProjects(String name, String subProjects) {
+    return '\"$name\" is deleted with its sub-projects $subProjects, and their sections and tasks, on all your devices.';
+  }
+
   @override
   String get projectEmptyTitle => 'Nothing to do here';
 
@@ -995,4 +1018,10 @@ class AppLocalizationsEnGb extends AppLocalizationsEn {
 /// The translations for English, as used in the United States (`en_US`).
 class AppLocalizationsEnUs extends AppLocalizationsEn {
   AppLocalizationsEnUs() : super('en_US');
+
+  @override
+  String get projectFavorite => 'Favorite';
+
+  @override
+  String get favoritesTitle => 'Favorites';
 }

apps/todo/halkyon_todo_flutter/lib/src/app/todo_dependencies.dart

@@ -14,6 +14,7 @@ import 'package:halkyon_todo_flutter/src/feature/label/data/label_repository.dar
 import 'package:halkyon_todo_flutter/src/feature/label/data/sync_label_gateway.dart';
 import 'package:halkyon_todo_flutter/src/feature/project/data/project_repository.dart';
 import 'package:halkyon_todo_flutter/src/feature/project/data/sync_project_gateway.dart';
+import 'package:halkyon_todo_flutter/src/feature/project/model/project_tree.dart';
 import 'package:halkyon_todo_flutter/src/feature/reminder/data/reminder_gateway.dart';
 import 'package:halkyon_todo_flutter/src/feature/reminder/data/system_reminder_gateway.dart';
 import 'package:halkyon_todo_flutter/src/feature/section/data/section_repository.dart';
@@ -62,14 +63,18 @@ class TodoPerson(
   final ReminderGateway reminders = const NoReminders(),
   final Future<void> Function()? _onDispose,
 }) implements Disposable {
-  /// Deletes the project [id] with its sections and tasks; throws
+  /// Deletes the project [id] with its sections and tasks, and its
+  /// sub-projects with theirs, as Todoist does (ADR 0033, D28); throws
   /// [InboxKept] for the Inbox, which keeps everything.
   Future<void> deleteProject(String id) async {
-    await projects.delete(id);
+    final ids = [id, ...descendantsOf(await projects.all(), id)];
-    await tasks.deleteIn(id);
+    for (final project in ids) {
+      await projects.delete(project);
+      await tasks.deleteIn(project);
+    }
     await sections.delete([
       for (final section in await sections.all())
-        if (section.content?.projectId == id) section.id,
+        if (ids.contains(section.content?.projectId)) section.id,
     ]);
   }
 

apps/todo/halkyon_todo_flutter/lib/src/feature/project/data/project_payload.dart

@@ -6,18 +6,42 @@ import 'package:halkyon_todo_flutter/src/feature/project/model/project_view.dart
 import 'package:halkyon_todo_flutter/src/feature/task/model/task.dart';
 import 'package:halkyon_ui/halkyon_ui.dart';
 
-/// The schema of a project's payload this app writes and reads.
+/// The schema of a project's payload this app writes and reads. It stays 1
+/// as fields are added (D17, D28): an older version reads a project with
+/// them, and keeps them as it writes the project once it knows to
+/// ([ProjectContent.unknown], since D28).
 const projectSchemaVersion = 1;
 
+/// The fields of a project's payload this version reads; any other is
+/// kept as it was (D28).
+const _known = {
+  'schemaVersion',
+  'name',
+  'color',
+  'position',
+  'view',
+  'description',
+  'parentId',
+  'favorite',
+  'defaultView',
+};
+
 /// [content] as its row's payload, before sealing: UTF-8 JSON, the colour by
-/// its palette name.
+/// its palette name; what a newer version wrote and this one doesn't know
+/// as it was, beneath what this one writes.
 ByteData encodeProject(ProjectContent content) =>
     encodeJson(projectSchemaVersion, {
+      ...content.unknown,
       'name': content.name,
       'color': ?content.color?.name,
       'position': content.position,
       // As at first, nothing: a project's Display is written once set (D17).
       if (!content.view.isDefault) 'view': encodeView(content.view),
+      if (content.description.isNotEmpty) 'description': content.description,
+      'parentId': ?content.parentId,
+      if (content.favorite) 'favorite': true,
+      if (!content.defaultView.isDefault)
+        'defaultView': encodeView(content.defaultView),
     });
 
 /// [view] by its values' names, those left as at first left out.
@@ -62,7 +86,9 @@ ProjectView decodeView(Object? json) {
 }
 
 /// The project [payload] holds, opened; null for one of a newer schema or one
-/// it can't read. A colour it doesn't know is none.
+/// it can't read. A colour it doesn't know is none; a payload of a version
+/// before D28 is a project of its own, no favourite, without a
+/// description, its Display's defaults as at first.
 ProjectContent? decodeProject(ByteData payload) => decodeJson(
   payload,
   version: projectSchemaVersion,
@@ -72,5 +98,13 @@ ProjectContent? decodeProject(ByteData payload) => decodeJson(
     color: PaletteColor.byName(json['color'] as String?),
     position: (json['position'] as num?)?.toDouble() ?? 0,
     view: decodeView(json['view']),
+    description: json['description'] as String? ?? '',
+    parentId: json['parentId'] as String?,
+    favorite: json['favorite'] == true,
+    defaultView: decodeView(json['defaultView']),
+    unknown: {
+      for (final MapEntry(:key, :value) in json.entries)
+        if (!_known.contains(key)) key: value,
+    },
   ),
 );

apps/todo/halkyon_todo_flutter/lib/src/feature/project/model/project.dart

@@ -5,9 +5,11 @@ import 'package:halkyon_ui/halkyon_ui.dart';
 /// What a project is (ADR 0033, D6), as its sealed payload holds it: its
 /// [name], its [color] from halkyon_ui's palette (D9; none: the theme's),
 /// its [position] among the person's projects, smaller first, and how it
-/// shows its tasks ([view], D17). The Inbox
+/// shows its tasks ([view], D17); since D28 its [description], the project
-/// is told by its row's ID, not by anything here (`ProjectRepository`), and
+/// it is a sub-project of ([parentId]), whether it is a [favorite], and
-/// shows under the app's own word for it, whatever [name] holds.
+/// the Display its Reset returns to ([defaultView]). The Inbox is told by
+/// its row's ID, not by anything here (`ProjectRepository`), and shows
+/// under the app's own word for it, whatever [name] holds.
 final class const ProjectContent({
   required final String name,
   final PaletteColor? color,
@@ -15,22 +17,65 @@ final class const ProjectContent({
 
   /// How the project shows its tasks (D17).
   final ProjectView view = const ProjectView(),
+
+  /// What the project is for, below its name in its editor (D28).
+  final String description = '',
+
+  /// The project it is a sub-project of (D28); none: a project of its own.
+  final String? parentId,
+
+  /// Whether Browse lists it among the Favorites too (D28).
+  final bool favorite = false,
+
+  /// The Display it is made with and its Reset returns to (D28): its
+  /// layout as its editor picks it, the rest as at first.
+  final ProjectView defaultView = const ProjectView(),
+
+  /// What its payload holds that this version doesn't know (as a task's,
+  /// D18): written back as it was, so that a project changed here keeps
+  /// what a newer version gave it.
+  final Map<String, Object?> unknown = const {},
 }) with Equatable {
   /// The Inbox's content, as the first device writes it.
   static const inbox = ProjectContent(name: '');
 
+  /// The longest name its editor takes (D28, as Todoist's).
+  static const maxNameLength = 120;
+
+  /// Whether its Display is the one it was made with (D28).
+  bool get viewIsDefault => view == defaultView;
+
   ProjectContent copyWith({
     String? name,
     PaletteColor? Function()? color,
     double? position,
     ProjectView? view,
+    String? description,
+    String? Function()? parentId,
+    bool? favorite,
+    ProjectView? defaultView,
   }) => ProjectContent(
     name: name ?? this.name,
     color: color == null ? this.color : color(),
     position: position ?? this.position,
     view: view ?? this.view,
+    description: description ?? this.description,
+    parentId: parentId == null ? this.parentId : parentId(),
+    favorite: favorite ?? this.favorite,
+    defaultView: defaultView ?? this.defaultView,
+    unknown: unknown,
   );
 
   @override
-  List<Object?> get props => [name, color, position, view];
+  List<Object?> get props => [
+    name,
+    color,
+    position,
+    view,
+    description,
+    parentId,
+    favorite,
+    defaultView,
+    unknown,
+  ];
 }

apps/todo/halkyon_todo_flutter/lib/src/feature/project/model/project_tree.dart

@@ -0,0 +1,85 @@
+import 'package:halkyon_todo_flutter/src/feature/project/model/stored_project.dart';
+
+/// A project as Browse lists it (ADR 0033, D28): how deep it stands under
+/// the projects it is a sub-project of, and whether it has sub-projects.
+typedef ProjectNode = ({StoredProject project, int depth, bool hasChildren});
+
+/// The person's own projects (not the Inbox, none that doesn't open) that
+/// can be listed: [projects] without those.
+List<StoredProject> _listed(List<StoredProject> projects) => [
+  for (final project in projects)
+    if (!project.inbox && project.content != null) project,
+];
+
+/// [projects] as Browse lists them (D28): each project of its own, in
+/// their order, each followed by its sub-projects, one level deeper. A
+/// project whose parent isn't listed (deleted, not opening here, or the
+/// Inbox) stands as a project of its own; so do projects whose parents
+/// run in a circle, as two devices could make them each moving one under
+/// the other, so that none is lost.
+List<ProjectNode> projectTree(List<StoredProject> projects) {
+  final listed = _listed(projects);
+  final ids = {for (final project in listed) project.id};
+  final children = <String, List<StoredProject>>{};
+  for (final project in listed) {
+    if (project.content!.parentId case final parent?
+        when ids.contains(parent)) {
+      (children[parent] ??= []).add(project);
+    }
+  }
+  final tree = <ProjectNode>[];
+  final placed = <String>{};
+  void place(StoredProject project, int depth) {
+    if (!placed.add(project.id)) {
+      return;
+    }
+    final below = children[project.id] ?? const [];
+    tree.add((project: project, depth: depth, hasChildren: below.isNotEmpty));
+    for (final child in below) {
+      place(child, depth + 1);
+    }
+  }
+
+  for (final project in listed) {
+    if (!ids.contains(project.content!.parentId)) {
+      place(project, 0);
+    }
+  }
+  // What is left stands in a circle: each a project of its own.
+  for (final project in listed) {
+    place(project, 0);
+  }
+  return tree;
+}
+
+/// The IDs of the project [id]'s sub-projects, theirs too, at every depth.
+Set<String> descendantsOf(List<StoredProject> projects, String id) {
+  final found = <String>{};
+  var frontier = {id};
+  while (frontier.isNotEmpty) {
+    frontier = {
+      for (final project in projects)
+        if (frontier.contains(project.content?.parentId) &&
+            project.id != id &&
+            found.add(project.id))
+          project.id,
+    };
+  }
+  return found;
+}
+
+/// The projects the project [id] can be moved under (D28): every one of
+/// the person's own but itself and its sub-projects at every depth, which
+/// would make it its own parent; for a new project ([id] null), every one.
+List<StoredProject> parentChoices(List<StoredProject> projects, String? id) {
+  final excluded = id == null
+      ? const <String>{}
+      : {
+          id,
+          ...descendantsOf(projects, id),
+        };
+  return [
+    for (final node in projectTree(projects))
+      if (!excluded.contains(node.project.id)) node.project,
+  ];
+}

apps/todo/halkyon_todo_flutter/lib/src/feature/project/widget/display_sheet.dart

@@ -69,25 +69,23 @@ DisplayTarget projectDisplay(String projectId) {
   return DisplayTarget(
     viewOf: viewOf,
     write: write,
-    isDefault: (state) => viewOf(state).isDefault,
+    // Reset returns to the Display the project was made with (D28).
-    reset: (todo, state) => write(todo, state, const ProjectView()),
+    isDefault: (state) =>
+        state.projectOf(projectId)?.content?.viewIsDefault ?? true,
+    reset: (todo, state) => write(
+      todo,
+      state,
+      state.projectOf(projectId)?.content?.defaultView ?? const ProjectView(),
+    ),
     grouping: (state) => !calendar(state),
     occurrences: calendar,
     layout: (context, state, apply) {
       final l10n = AppLocalizations.of(context);
       final view = viewOf(state);
       return [
-        _LayoutPicker<ProjectLayout>(
+        LayoutPicker<ProjectLayout>(
           selected: view.layout,
-          segments: [
+          segments: projectLayouts(l10n),
-            (ProjectLayout.list, LucideIcons.list, l10n.layoutList),
-            (ProjectLayout.board, LucideIcons.squareKanban, l10n.layoutBoard),
-            (
-              ProjectLayout.calendar,
-              LucideIcons.calendarDays,
-              l10n.layoutCalendar,
-            ),
-          ],
           onPicked: (layout) => apply(
             (todo) => write(todo, state, view.copyWith(layout: layout)),
           ),
@@ -125,7 +123,7 @@ final upcomingDisplay = DisplayTarget(
       (todo) => todo.settings.write(state.settings.copyWith(upcoming: view)),
     );
     return [
-      _LayoutPicker<UpcomingLayout>(
+      LayoutPicker<UpcomingLayout>(
         selected: upcoming.layout,
         segments: [
           (UpcomingLayout.agenda, LucideIcons.list, l10n.layoutAgenda),
@@ -257,8 +255,8 @@ class const _DisplaySheet({required final DisplayTarget target})
         crossAxisAlignment: CrossAxisAlignment.stretch,
         children: [
           heading(l10n.layoutTitle),
-          _Group(children: target.layout(context, state, apply)),
+          RoundedGroup(children: target.layout(context, state, apply)),
-          _Group(
+          RoundedGroup(
             children: [
               SwitchListTile(
                 secondary: const Icon(LucideIcons.circleCheck),
@@ -276,7 +274,7 @@ class const _DisplaySheet({required final DisplayTarget target})
             ],
           ),
           heading(l10n.displaySort),
-          _Group(
+          RoundedGroup(
             children: [
               if (target.grouping(state))
                 choice(
@@ -316,7 +314,7 @@ class const _DisplaySheet({required final DisplayTarget target})
             ],
           ),
           heading(l10n.displayFilter),
-          _Group(
+          RoundedGroup(
             children: [
               if (target.dateFilter)
                 choice(
@@ -412,10 +410,22 @@ class const _DisplaySheet({required final DisplayTarget target})
 /// 3 Nov 2026): each a phone's outline with its layout's picture in it,
 /// its name below on one line, and a radio button marking the one in use;
 /// the picture, the name or the button picks it.
-class const _LayoutPicker<T>({
+/// A project's layouts as a [LayoutPicker] offers them: List, Board,
+/// Calendar, each with its picture.
+List<(ProjectLayout, IconData, String)> projectLayouts(AppLocalizations l10n) =>
+    [
+      (ProjectLayout.list, LucideIcons.list, l10n.layoutList),
+      (ProjectLayout.board, LucideIcons.squareKanban, l10n.layoutBoard),
+      (ProjectLayout.calendar, LucideIcons.calendarDays, l10n.layoutCalendar),
+    ];
+
+/// Layouts side by side, as Todoist's (D22): each a phone's outline with its
+/// picture, its name below, a radio button marking the [selected] one.
+class const LayoutPicker<T>({
   required final T selected,
   required final List<(T, IconData, String)> segments,
   required final ValueChanged<T> onPicked,
+  super.key,
 }) extends StatelessWidget {
   @override
   Widget build(BuildContext context) {
@@ -481,8 +491,9 @@ class const _LayoutPicker<T>({
 }
 
 /// [children] in a box rounded at its ends, as Todoist's Display groups
-/// its rows (the developer's review, 3 Nov 2026).
+/// its rows (the developer's review, 3 Nov 2026), and a project's editor
-class const _Group({required final List<Widget> children})
+/// its own (D28).
+class const RoundedGroup({required final List<Widget> children, super.key})
     extends StatelessWidget {
   @override
   Widget build(BuildContext context) => Padding(

apps/todo/halkyon_todo_flutter/lib/src/feature/project/widget/project_dialogs.dart

@@ -1,35 +1,9 @@
 import 'package:halkyon_todo_flutter/l10n/app_localizations.dart';
 import 'package:halkyon_todo_flutter/src/feature/label/model/label.dart';
-import 'package:halkyon_todo_flutter/src/feature/project/model/project.dart';
 import 'package:halkyon_todo_flutter/src/feature/section/model/section.dart';
 import 'package:halkyon_ui/halkyon_ui.dart';
 import 'package:material_ui/material_ui.dart';
 
-/// Asks for a project's name and colour (ADR 0033, D9), [current]'s to
-/// change, or a new one's; null where the person changed their mind.
-Future<ProjectContent?> editProject(
-  BuildContext context, {
-  ProjectContent? current,
-}) async {
-  final l10n = AppLocalizations.of(context);
-  final named = await _nameAndColor(
-    context,
-    title: current == null ? l10n.projectAdd : l10n.projectEdit,
-    name: current?.name ?? '',
-    color: current?.color ?? PaletteColor.cobalt,
-  );
-  return switch (named) {
-    // The rest kept as it is: building it anew had reset the project's
-    // Display (the developer's review, 4 Nov 2026).
-    (:final name, :final color) =>
-      (current ?? ProjectContent(name: name)).copyWith(
-        name: name,
-        color: () => color,
-      ),
-    null => null,
-  };
-}
-
 /// Asks for a label's name and colour, [current]'s to change, or a new
 /// one's; null where the person changed their mind.
 Future<LabelContent?> editLabel(

apps/todo/halkyon_todo_flutter/lib/src/feature/project/widget/project_editor.dart

@@ -0,0 +1,299 @@
+import 'package:halkyon_todo_flutter/l10n/app_localizations.dart';
+import 'package:halkyon_todo_flutter/src/feature/project/model/project.dart';
+import 'package:halkyon_todo_flutter/src/feature/project/model/project_tree.dart';
+import 'package:halkyon_todo_flutter/src/feature/project/model/project_view.dart';
+import 'package:halkyon_todo_flutter/src/feature/project/model/stored_project.dart';
+import 'package:halkyon_todo_flutter/src/feature/project/widget/display_sheet.dart';
+import 'package:halkyon_todo_flutter/src/feature/task/widget/task_texts.dart';
+import 'package:halkyon_ui/halkyon_ui.dart';
+import 'package:lucide_icons_flutter/lucide_icons.dart';
+import 'package:material_ui/material_ui.dart';
+
+/// Asks for a project as Todoist's Add project does (ADR 0033, D28): its
+/// name, description, colour, parent among [projects], whether it is a
+/// favourite, and its layout; [current]'s, the project [id], to change, or a
+/// new one's. Null where the person changed their mind.
+///
+/// A screen of its own on a phone, with ← and ✓ in its app bar; where
+/// sheets are dialogs (from expanded windows on, and on the desktop), the
+/// same in a dialog, as the app's other editors.
+Future<ProjectContent?> editProject(
+  BuildContext context, {
+  required List<StoredProject> projects,
+  String? id,
+  ProjectContent? current,
+}) {
+  final editor = _ProjectEditor(id: id, current: current, projects: projects);
+  if (sheetShowsAsDialog(context)) {
+    return showDialog<ProjectContent>(
+      context: context,
+      builder: (context) => Dialog(
+        clipBehavior: Clip.antiAlias,
+        child: ConstrainedBox(
+          constraints: const BoxConstraints(maxWidth: 560, maxHeight: 640),
+          child: editor,
+        ),
+      ),
+    );
+  }
+  return Navigator.of(context, rootNavigator: true).push(
+    MaterialPageRoute<ProjectContent>(
+      fullscreenDialog: true,
+      builder: (context) => editor,
+    ),
+  );
+}
+
+class const _ProjectEditor({
+  required final String? id,
+  required final ProjectContent? current,
+  required final List<StoredProject> projects,
+}) extends StatefulWidget {
+  @override
+  State<_ProjectEditor> createState() => _ProjectEditorState();
+}
+
+class _ProjectEditorState() extends State<_ProjectEditor> {
+  late final _name = TextEditingController(text: widget.current?.name);
+  late final _description = TextEditingController(
+    text: widget.current?.description,
+  );
+  late var _color = widget.current?.color ?? PaletteColor.cobalt;
+  late var _parentId = widget.current?.parentId;
+  late var _favorite = widget.current?.favorite ?? false;
+
+  /// The layout it is made with, which its Display's Reset returns to.
+  late var _layout = widget.current?.defaultView.layout ?? ProjectLayout.list;
+
+  @override
+  void initState() {
+    super.initState();
+    // ✓ is on once there is a name.
+    _name.addListener(() => setState(() {}));
+  }
+
+  @override
+  void dispose() {
+    _name.dispose();
+    _description.dispose();
+    super.dispose();
+  }
+
+  void _save() {
+    final name = _name.text.trim();
+    if (name.isEmpty) {
+      return;
+    }
+    final description = _description.text.trim();
+    final current = widget.current;
+    Navigator.of(context).pop(switch (current) {
+      // A new project starts with its layout (D28).
+      null => ProjectContent(
+        name: name,
+        color: _color,
+        description: description,
+        parentId: _parentId,
+        favorite: _favorite,
+        view: ProjectView(layout: _layout),
+        defaultView: ProjectView(layout: _layout),
+      ),
+      // The rest kept as it is, its Display too (the developer's review,
+      // 4 Nov 2026); a layout picked anew shows at once.
+      final current => current.copyWith(
+        name: name,
+        color: () => _color,
+        description: description,
+        parentId: () => _parentId,
+        favorite: _favorite,
+        view: _layout == current.defaultView.layout
+            ? null
+            : current.view.copyWith(layout: _layout),
+        defaultView: current.defaultView.copyWith(layout: _layout),
+      ),
+    });
+  }
+
+  StoredProject? get _parent => switch (_parentId) {
+    final id? => widget.projects.where((p) => p.id == id).firstOrNull,
+    null => null,
+  };
+
+  Future<void> _pickColor() async {
+    final l10n = AppLocalizations.of(context);
+    final picked = await showDialog<PaletteColor>(
+      context: context,
+      builder: (context) => AlertDialog(
+        title: Text(l10n.colorLabel),
+        content: SizedBox(
+          width: 320,
+          child: PaletteColorPicker(
+            selected: _color,
+            onChanged: (color) => Navigator.of(context).pop(color),
+          ),
+        ),
+      ),
+    );
+    if (picked != null) {
+      setState(() => _color = picked);
+    }
+  }
+
+  Future<void> _pickParent() async {
+    final picked = await pickParentProject(
+      context,
+      projects: widget.projects,
+      id: widget.id,
+      current: _parentId,
+    );
+    if (picked case (:final parentId)) {
+      setState(() => _parentId = parentId);
+    }
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    final l10n = AppLocalizations.of(context);
+    final ui = HalkyonUiLocalizations.of(context);
+    final theme = Theme.of(context);
+    final muted = theme.colorScheme.onSurfaceVariant;
+    final canSave = _name.text.trim().isNotEmpty;
+    return Scaffold(
+      appBar: AppBar(
+        leading: IconButton(
+          tooltip: MaterialLocalizations.of(context).backButtonTooltip,
+          icon: const Icon(LucideIcons.arrowLeft),
+          onPressed: () => Navigator.of(context).pop(),
+        ),
+        title: Text(
+          widget.current == null ? l10n.projectAdd : l10n.projectEdit,
+        ),
+        actions: [
+          IconButton(
+            tooltip: l10n.save,
+            icon: const Icon(LucideIcons.check),
+            onPressed: canSave ? _save : null,
+          ),
+          const SizedBox(width: 4),
+        ],
+      ),
+      body: ListView(
+        padding: const EdgeInsets.only(bottom: 24),
+        children: [
+          Padding(
+            padding: const EdgeInsets.fromLTRB(16, 8, 16, 0),
+            child: TextField(
+              controller: _name,
+              autofocus: widget.current == null,
+              maxLength: ProjectContent.maxNameLength,
+              textCapitalization: TextCapitalization.sentences,
+              decoration: InputDecoration(labelText: l10n.nameLabel),
+              onSubmitted: (_) => _save(),
+            ),
+          ),
+          Padding(
+            padding: const EdgeInsets.fromLTRB(16, 4, 16, 8),
+            child: TextField(
+              controller: _description,
+              minLines: 1,
+              maxLines: 5,
+              textCapitalization: TextCapitalization.sentences,
+              decoration: InputDecoration(labelText: l10n.taskDescription),
+            ),
+          ),
+          RoundedGroup(
+            children: [
+              ListTile(
+                leading: Icon(
+                  LucideIcons.palette,
+                  color: paletteText(context, _color),
+                ),
+                title: Text(l10n.colorLabel),
+                trailing: Text(
+                  ui.paletteColorName(_color.name),
+                  style: TextStyle(color: muted),
+                ),
+                onTap: _pickColor,
+              ),
+              ListTile(
+                leading: const Icon(LucideIcons.folderTree),
+                title: Text(l10n.projectParent),
+                trailing: ConstrainedBox(
+                  constraints: const BoxConstraints(maxWidth: 160),
+                  child: Text(
+                    _parent?.content?.name ?? l10n.projectNoParent,
+                    maxLines: 1,
+                    overflow: TextOverflow.ellipsis,
+                    style: TextStyle(color: muted),
+                  ),
+                ),
+                onTap: _pickParent,
+              ),
+              SwitchListTile(
+                secondary: const Icon(LucideIcons.heart),
+                title: Text(l10n.projectFavorite),
+                value: _favorite,
+                onChanged: (on) => setState(() => _favorite = on),
+              ),
+            ],
+          ),
+          Padding(
+            padding: const EdgeInsets.fromLTRB(16, 16, 16, 4),
+            child: Text(l10n.layoutTitle, style: theme.textTheme.titleSmall),
+          ),
+          RoundedGroup(
+            children: [
+              LayoutPicker<ProjectLayout>(
+                selected: _layout,
+                segments: projectLayouts(l10n),
+                onPicked: (layout) => setState(() => _layout = layout),
+              ),
+            ],
+          ),
+        ],
+      ),
+    );
+  }
+}
+
+/// Asks under which project the project [id] (none: a new one) goes, none
+/// for a project of its own, [current] ticked: every project of the
+/// person's but itself and its sub-projects (D28), indented as Browse
+/// lists them. Null where the person changed their mind.
+Future<({String? parentId})?> pickParentProject(
+  BuildContext context, {
+  required List<StoredProject> projects,
+  required String? id,
+  required String? current,
+}) => showAdaptiveSheet<({String? parentId})>(
+  context,
+  builder: (context) {
+    final l10n = AppLocalizations.of(context);
+    final choices = {for (final p in parentChoices(projects, id)) p.id};
+    Widget option(String? parentId, String name, {int depth = 0}) => ListTile(
+      contentPadding: EdgeInsetsDirectional.only(start: 16 + 24.0 * depth),
+      leading: Icon(
+        parentId == null ? LucideIcons.circleSlash : LucideIcons.hash,
+      ),
+      title: Text(name),
+      trailing: parentId == current ? const Icon(LucideIcons.check) : null,
+      onTap: () =>
+          Navigator.of(context).pop<({String? parentId})>((parentId: parentId)),
+    );
+    return SheetScaffold(
+      title: l10n.projectParent,
+      body: Column(
+        mainAxisSize: MainAxisSize.min,
+        children: [
+          option(null, l10n.projectNoParent),
+          for (final node in projectTree(projects))
+            if (choices.contains(node.project.id))
+              option(
+                node.project.id,
+                node.project.content!.name,
+                depth: node.depth,
+              ),
+        ],
+      ),
+    );
+  },
+);

apps/todo/halkyon_todo_flutter/lib/src/feature/project/widget/project_screen.dart

@@ -4,8 +4,9 @@ import 'package:halkyon_app/halkyon_app.dart';
 import 'package:halkyon_todo_flutter/l10n/app_localizations.dart';
 import 'package:halkyon_todo_flutter/src/app/todo_dependencies.dart';
 import 'package:halkyon_todo_flutter/src/core/widget/menu_entry.dart';
+import 'package:halkyon_todo_flutter/src/feature/project/model/project_tree.dart';
 import 'package:halkyon_todo_flutter/src/feature/project/widget/display_sheet.dart';
-import 'package:halkyon_todo_flutter/src/feature/project/widget/project_dialogs.dart';
+import 'package:halkyon_todo_flutter/src/feature/project/widget/project_editor.dart';
 import 'package:halkyon_todo_flutter/src/feature/project/widget/project_tasks.dart';
 import 'package:halkyon_todo_flutter/src/feature/task/bloc/todo_cubit.dart';
 import 'package:halkyon_todo_flutter/src/feature/task/widget/task_drag.dart';
@@ -110,7 +111,12 @@ class const ProjectScreen({required final String projectId, super.key})
               PopupMenuItem(
                 value: () async {
                   final todo = PersonScope.of<TodoPerson>(context);
-                  if (await editProject(context, current: content)
+                  if (await editProject(
+                        context,
+                        projects: state.projects,
+                        id: projectId,
+                        current: content,
+                      )
                       case final edited?) {
                     await todo.projects.update(projectId, edited);
                   }
@@ -124,7 +130,22 @@ class const ProjectScreen({required final String projectId, super.key})
                   if (await confirmDestructiveAction(
                     context,
                     title: l10n.projectDeleteTitle,
-                    message: l10n.projectDeleteMessage(content.name),
+                    // Its sub-projects go with it, as Todoist's (D28),
+                    // named here first.
+                    message: switch ([
+                      for (final node in projectTree(state.projects))
+                        if (descendantsOf(
+                          state.projects,
+                          projectId,
+                        ).contains(node.project.id))
+                          node.project.content!.name,
+                    ]) {
+                      [] => l10n.projectDeleteMessage(content.name),
+                      final names => l10n.projectDeleteWithSubProjects(
+                        content.name,
+                        names.join(', '),
+                      ),
+                    },
                     confirmLabel: l10n.projectDelete,
                   )) {
                     await todo.deleteProject(projectId);

apps/todo/halkyon_todo_flutter/lib/src/tabs/browse_tab.dart

@@ -4,7 +4,10 @@ import 'package:halkyon_app/halkyon_app.dart';
 import 'package:halkyon_todo_flutter/l10n/app_localizations.dart';
 import 'package:halkyon_todo_flutter/src/app/todo_dependencies.dart';
 import 'package:halkyon_todo_flutter/src/app/todo_paths.dart';
+import 'package:halkyon_todo_flutter/src/feature/project/model/project_tree.dart';
+import 'package:halkyon_todo_flutter/src/feature/project/model/stored_project.dart';
 import 'package:halkyon_todo_flutter/src/feature/project/widget/project_dialogs.dart';
+import 'package:halkyon_todo_flutter/src/feature/project/widget/project_editor.dart';
 import 'package:halkyon_todo_flutter/src/feature/task/bloc/todo_cubit.dart';
 import 'package:halkyon_todo_flutter/src/feature/task/model/task_views.dart';
 import 'package:halkyon_todo_flutter/src/feature/task/widget/task_texts.dart';
@@ -19,8 +22,19 @@ import 'package:material_ui/material_ui.dart';
 /// person's projects, each in its colour with
 /// how many of its tasks are open, one opening its screen, and one more to
 /// add; their labels, to add, rename or delete; the cog to Settings. What
-/// doesn't open on this device is counted at the foot.
+/// doesn't open on this device is counted at the foot. Since D28, as
-class const BrowseTab({super.key}) extends StatelessWidget {
+/// Todoist's: the favourite projects in a group of their own above, and
+/// each project's sub-projects indented under it, folding away under its
+/// chevron (not kept: each start shows them all).
+class const BrowseTab({super.key}) extends StatefulWidget {
+  @override
+  State<BrowseTab> createState() => _BrowseTabState();
+}
+
+class _BrowseTabState() extends State<BrowseTab> {
+  /// The projects whose sub-projects are folded away.
+  final _folded = <String>{};
+
   @override
   Widget build(BuildContext context) {
     final l10n = AppLocalizations.of(context);
@@ -47,6 +61,53 @@ class const BrowseTab({super.key}) extends StatelessWidget {
       for (final project in state.projects)
         if (!project.inbox && project.content != null) project,
     ];
+    final tree = projectTree(state.projects);
+    final favorites = [
+      for (final project in projects)
+        if (project.content!.favorite) project,
+    ];
+    // Hidden: those under a folded project, however deep.
+    final hidden = {
+      for (final id in _folded) ...descendantsOf(state.projects, id),
+    };
+    Widget projectRow(StoredProject project, {int depth = 0, bool? folded}) =>
+        ListTile(
+          contentPadding: EdgeInsetsDirectional.only(
+            start: 16 + 24.0 * depth,
+            end: folded == null ? 16 : 4,
+          ),
+          leading: Icon(
+            LucideIcons.hash,
+            color: switch (project.content!.color) {
+              final color? => paletteText(context, color),
+              null => null,
+            },
+          ),
+          title: Text(project.content!.name),
+          trailing: Row(
+            mainAxisSize: MainAxisSize.min,
+            children: [
+              if (openCountIn(state.tasks, project.id) case final count
+                  when count > 0)
+                Text('$count'),
+              if (folded != null)
+                IconButton(
+                  tooltip: folded
+                      ? l10n.projectSubProjectsShow
+                      : l10n.projectSubProjectsHide,
+                  icon: Icon(
+                    folded ? LucideIcons.chevronRight : LucideIcons.chevronDown,
+                  ),
+                  onPressed: () => setState(
+                    () => folded
+                        ? _folded.remove(project.id)
+                        : _folded.add(project.id),
+                  ),
+                ),
+            ],
+          ),
+          onTap: () => context.go(TodoPaths.project(project.id)),
+        );
     return TabScreen(
       title: l10n.shellTabBrowse,
       actions: const [SettingsButton()],
@@ -77,6 +138,26 @@ class const BrowseTab({super.key}) extends StatelessWidget {
                       ),
                     ),
                     const SizedBox(height: 12),
+                    if (favorites.isNotEmpty) ...[
+                      SectionCard(
+                        padding: const EdgeInsets.only(bottom: 8),
+                        child: Column(
+                          crossAxisAlignment: CrossAxisAlignment.stretch,
+                          children: [
+                            Padding(
+                              padding: const EdgeInsets.fromLTRB(16, 16, 16, 8),
+                              child: Text(
+                                l10n.favoritesTitle,
+                                style: theme.textTheme.titleMedium,
+                              ),
+                            ),
+                            for (final project in favorites)
+                              projectRow(project),
+                          ],
+                        ),
+                      ),
+                      const SizedBox(height: 12),
+                    ],
                     SectionCard(
                       padding: const EdgeInsets.only(bottom: 8),
                       child: Column(
@@ -86,7 +167,10 @@ class const BrowseTab({super.key}) extends StatelessWidget {
                             l10n.projectsTitle,
                             tip: l10n.projectAdd,
                             onAdd: () async {
-                              final added = await editProject(context);
+                              final added = await editProject(
+                                context,
+                                projects: state.projects,
+                              );
                               if (added != null) {
                                 await todo.projects.save(
                                   added.copyWith(
@@ -108,26 +192,15 @@ class const BrowseTab({super.key}) extends StatelessWidget {
                                 ),
                               ),
                             ),
-                          for (final project in projects)
+                          for (final node in tree)
-                            ListTile(
+                            if (!hidden.contains(node.project.id))
-                              leading: Icon(
+                              projectRow(
-                                LucideIcons.hash,
+                                node.project,
-                                color: switch (project.content!.color) {
+                                depth: node.depth,
-                                  final color? => paletteText(context, color),
+                                folded: node.hasChildren
-                                  null => null,
+                                    ? _folded.contains(node.project.id)
-                                },
+                                    : null,
                               ),
-                              title: Text(project.content!.name),
-                              trailing: switch (openCountIn(
-                                state.tasks,
-                                project.id,
-                              )) {
-                                0 => null,
-                                final count => Text('$count'),
-                              },
-                              onTap: () =>
-                                  context.go(TodoPaths.project(project.id)),
-                            ),
                         ],
                       ),
                     ),

apps/todo/halkyon_todo_flutter/test/app/todo_app_test.dart

@@ -13,6 +13,7 @@ import 'package:halkyon_todo_flutter/src/feature/label/model/label.dart';
 import 'package:halkyon_todo_flutter/src/feature/project/data/project_repository.dart';
 import 'package:halkyon_todo_flutter/src/feature/project/model/project.dart';
 import 'package:halkyon_todo_flutter/src/feature/project/model/project_view.dart';
+import 'package:halkyon_todo_flutter/src/feature/project/model/stored_project.dart';
 import 'package:halkyon_todo_flutter/src/feature/project/widget/month_grid.dart';
 import 'package:halkyon_todo_flutter/src/feature/project/widget/project_board.dart';
 import 'package:halkyon_todo_flutter/src/feature/project/widget/project_calendar_view.dart';
@@ -1966,10 +1967,11 @@ void main() {
       await start(tester, usedBefore: true);
       await goTab(tester, 'Browse');
 
-      await tester.tap(find.byTooltip('New project'));
+      await tester.tap(find.byTooltip('Add project'));
       await tester.pumpAndSettle();
-      await tester.enterText(find.byType(TextField), 'Home');
+      await tester.enterText(find.byType(TextField).first, 'Home');
-      await tester.tap(find.text('Save'));
+      await tester.pump();
+      await tester.tap(find.byTooltip('Save'));
       await tester.pumpAndSettle();
       expect(find.text('Home'), findsOneWidget);
 
@@ -2019,7 +2021,8 @@ void main() {
       await tester.tap(find.text('Edit project'));
       await tester.pumpAndSettle();
       await tester.enterText(find.byType(TextField).first, 'Yard');
-      await tester.tap(find.text('Save'));
+      await tester.pump();
+      await tester.tap(find.byTooltip('Save'));
       await tester.pumpAndSettle();
 
       final stored = await tester.runAsync(projects.all);
@@ -2028,6 +2031,175 @@ void main() {
       expect(yard?.view, const ProjectView(layout: ProjectLayout.board));
     });
 
+    group("a project's editor as Todoist's Add project (D28)", () {
+      ProjectRepository projects() => ProjectRepository(projectRows, keys);
+
+      /// Seeds Home with its sub-project Garden, and Work.
+      Future<Map<String, String>> seedProjects(WidgetTester tester) async {
+        final ids = await tester.runAsync(() async {
+          final home = await projects().save(
+            const ProjectContent(name: 'Home', position: 1),
+          );
+          final garden = await projects().save(
+            ProjectContent(name: 'Garden', position: 2, parentId: home),
+          );
+          final work = await projects().save(
+            const ProjectContent(name: 'Work', position: 3),
+          );
+          return {'Home': home, 'Garden': garden, 'Work': work};
+        });
+        projectRows.bringRemoteChanges();
+        await tester.pumpAndSettle();
+        return ids!;
+      }
+
+      Future<StoredProject> stored(WidgetTester tester, String name) async =>
+          (await tester.runAsync(projects().all))!.singleWhere(
+            (p) => p.content?.name == name,
+          );
+
+      testWidgets('a full screen with its name counted to 120, description, '
+          'colour, parent, favourite and layout; Browse lists it under its '
+          'parent and among the Favourites', (tester) async {
+        await start(tester, usedBefore: true);
+        final ids = await seedProjects(tester);
+        await goTab(tester, 'Browse');
+
+        await tester.tap(find.byTooltip('Add project'));
+        await tester.pumpAndSettle();
+        expect(find.widgetWithText(AppBar, 'Add project'), findsOneWidget);
+        expect(find.text('0/120'), findsOneWidget);
+        expect(
+          tester
+              .widget<IconButton>(
+                find.widgetWithIcon(IconButton, LucideIcons.check),
+              )
+              .onPressed,
+          isNull,
+          reason: 'no name yet',
+        );
+        await tester.enterText(find.byType(TextField).first, 'Roadmap');
+        await tester.enterText(find.byType(TextField).at(1), 'What comes');
+        await tester.pump();
+        expect(find.text('7/120'), findsOneWidget);
+        await tester.tap(find.text('Parent project'));
+        await tester.pumpAndSettle();
+        await tester.tap(find.widgetWithText(ListTile, 'Home').last);
+        await tester.pumpAndSettle();
+        expect(find.widgetWithText(ListTile, 'Parent project'), findsOneWidget);
+        // The test's language is American English.
+        await tester.tap(find.text('Favorite'));
+        await tester.ensureVisible(find.text('Board'));
+        await tester.pumpAndSettle();
+        await tester.tap(find.text('Board'));
+        await tester.pump();
+        await tester.tap(find.byTooltip('Save'));
+        await tester.pumpAndSettle();
+
+        final roadmap = (await stored(tester, 'Roadmap')).content!;
+        expect(roadmap.description, 'What comes');
+        expect(roadmap.parentId, ids['Home']);
+        expect(roadmap.favorite, isTrue);
+        expect(roadmap.view, const ProjectView(layout: ProjectLayout.board));
+        expect(roadmap.defaultView, roadmap.view);
+
+        expect(find.text('Favorites'), findsOneWidget);
+        expect(find.text('Roadmap'), findsNWidgets(2));
+        double startOf(String name) =>
+            tester.getTopLeft(find.text(name).last).dx;
+        expect(startOf('Garden'), greaterThan(startOf('Home')));
+        expect(startOf('Roadmap'), startOf('Garden'));
+        await tester.tap(find.byTooltip('Hide sub-projects'));
+        await tester.pumpAndSettle();
+        expect(find.text('Garden'), findsNothing);
+        expect(find.text('Roadmap'), findsOneWidget, reason: 'a favourite');
+        await tester.tap(find.byTooltip('Show sub-projects'));
+        await tester.pumpAndSettle();
+        expect(find.text('Garden'), findsOneWidget);
+      });
+
+      testWidgets('a project is never offered as its own parent, nor its '
+          'sub-projects', (tester) async {
+        await start(tester, usedBefore: true);
+        await seedProjects(tester);
+        await goTab(tester, 'Browse');
+        await tester.tap(find.text('Home'));
+        await tester.pumpAndSettle();
+        await tester.tap(find.byTooltip('More'));
+        await tester.pumpAndSettle();
+        await tester.tap(find.text('Edit project'));
+        await tester.pumpAndSettle();
+        expect(find.widgetWithText(AppBar, 'Edit project'), findsOneWidget);
+        await tester.tap(find.text('Parent project'));
+        await tester.pumpAndSettle();
+
+        final sheet = find.byType(SheetScaffold);
+        Finder offered(String name) =>
+            find.descendant(of: sheet, matching: find.text(name));
+        expect(offered('No parent'), findsOneWidget);
+        expect(offered('Work'), findsOneWidget);
+        expect(offered('Home'), findsNothing);
+        expect(offered('Garden'), findsNothing);
+      });
+
+      testWidgets("Display's Reset returns to the layout the project was made "
+          'with', (tester) async {
+        await start(tester, usedBefore: true);
+        await tester.runAsync(
+          () => projects().save(
+            const ProjectContent(
+              name: 'Roadmap',
+              color: PaletteColor.fern,
+              view: ProjectView(layout: ProjectLayout.board, completed: true),
+              defaultView: ProjectView(layout: ProjectLayout.board),
+            ),
+          ),
+        );
+        projectRows.bringRemoteChanges();
+        await goTab(tester, 'Browse');
+        await tester.tap(find.text('Roadmap'));
+        await tester.pumpAndSettle();
+        await tester.tap(find.byTooltip('Display'));
+        await tester.pumpAndSettle();
+        await tester.tap(find.text('Reset all'));
+        await tester.pumpAndSettle();
+
+        final roadmap = (await stored(tester, 'Roadmap')).content!;
+        expect(roadmap.view, const ProjectView(layout: ProjectLayout.board));
+        expect(roadmap.color, PaletteColor.fern);
+      });
+
+      testWidgets('deleting a project names its sub-projects first and takes '
+          'them with it, as Todoist', (tester) async {
+        await start(tester, usedBefore: true);
+        final ids = await seedProjects(tester);
+        await seed(
+          tester,
+          TaskContent(title: 'Weed', projectId: ids['Garden']!),
+        );
+        await goTab(tester, 'Browse');
+        await tester.tap(find.text('Home'));
+        await tester.pumpAndSettle();
+        await tester.tap(find.byTooltip('More'));
+        await tester.pumpAndSettle();
+        await tester.tap(find.text('Delete project'));
+        await tester.pumpAndSettle();
+        expect(find.textContaining('sub-projects Garden'), findsOneWidget);
+        await tester.tap(find.text('Delete project').last);
+        await tester.pumpAndSettle();
+
+        final left = await tester.runAsync(projects().all);
+        expect(
+          [
+            for (final p in left!)
+              if (!p.inbox) p.content?.name,
+          ],
+          ['Work'],
+        );
+        expect(await tester.runAsync(() => tasks().all()), isEmpty);
+      });
+    });
+
     testWidgets("a project's screen washes in its own colour, as a "
         "studyset's in Learn; one without keeps Todo's strawberry", (
       tester,

apps/todo/halkyon_todo_flutter/test/feature/project/project_payload_test.dart

@@ -107,4 +107,70 @@ void main() {
       isNull,
     );
   });
+
+  group('since D28', () {
+    Map<String, Object?> written(ByteData payload) =>
+        jsonDecode(utf8.decode(Uint8List.sublistView(payload)))
+            as Map<String, Object?>;
+
+    test('its description, parent, favourite and default Display read back '
+        'as written', () {
+      const garden = ProjectContent(
+        name: 'Garden',
+        description: 'Beds and the shed',
+        parentId: 'home',
+        favorite: true,
+        view: ProjectView(layout: ProjectLayout.board, completed: true),
+        defaultView: ProjectView(layout: ProjectLayout.board),
+      );
+      expect(decodeProject(encodeProject(garden)), garden);
+      expect(written(encodeProject(garden)), {
+        'schemaVersion': 1,
+        'name': 'Garden',
+        'position': 0.0,
+        'view': {'layout': 'board', 'completed': true},
+        'description': 'Beds and the shed',
+        'parentId': 'home',
+        'favorite': true,
+        'defaultView': {'layout': 'board'},
+      });
+    });
+
+    test('a payload from before reads as a project of its own, no '
+        'favourite, its Display defaults as at first', () {
+      final older = decodeProject(
+        json({
+          'schemaVersion': 1,
+          'name': 'Home',
+          'position': 1,
+          'view': {'layout': 'calendar'},
+        }),
+      );
+      expect(older?.description, '');
+      expect(older?.parentId, isNull);
+      expect(older?.favorite, isFalse);
+      expect(older?.defaultView, const ProjectView());
+      expect(older?.unknown, isEmpty);
+      expect(older?.viewIsDefault, isFalse, reason: 'its Display was set');
+    });
+
+    test('what a newer version wrote that this one doesn’t know is kept, '
+        'and written back as it was, however the project changes here', () {
+      final newer = {
+        'schemaVersion': 1,
+        'name': 'Home',
+        'position': 0.0,
+        'icon': 'house',
+        'members': ['a', 'b'],
+      };
+      final read = decodeProject(json(newer))!;
+      expect(read.unknown, {
+        'icon': 'house',
+        'members': ['a', 'b'],
+      });
+      final renamed = read.copyWith(name: 'House');
+      expect(written(encodeProject(renamed)), {...newer, 'name': 'House'});
+      expect(decodeProject(encodeProject(renamed)), renamed);
+    });
+  });
 }

apps/todo/halkyon_todo_flutter/test/feature/project/project_tree_test.dart

@@ -0,0 +1,66 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:halkyon_todo_flutter/src/feature/project/model/project.dart';
+import 'package:halkyon_todo_flutter/src/feature/project/model/project_tree.dart';
+import 'package:halkyon_todo_flutter/src/feature/project/model/stored_project.dart';
+
+/// Sub-projects (ADR 0033, D28): Browse lists each under its parent, and a
+/// project is never moved under itself or a project below it.
+void main() {
+  StoredProject project(String id, {String? parent}) =>
+      StoredProject(id, ProjectContent(name: id, parentId: parent));
+
+  final projects = [
+    const StoredProject('inbox', ProjectContent.inbox, inbox: true),
+    project('Home'),
+    project('Work'),
+    project('Garden', parent: 'Home'),
+    project('Beds', parent: 'Garden'),
+    project('Kitchen', parent: 'Home'),
+    const StoredProject('locked', null),
+  ];
+
+  test('each project is followed by its sub-projects, a level deeper, in '
+      'their order; the Inbox and what does not open are left out', () {
+    expect(
+      [
+        for (final node in projectTree(projects))
+          (node.project.id, node.depth, node.hasChildren),
+      ],
+      [
+        ('Home', 0, true),
+        ('Garden', 1, true),
+        ('Beds', 2, false),
+        ('Kitchen', 1, false),
+        ('Work', 0, false),
+      ],
+    );
+  });
+
+  test('a project whose parent is gone, or whose parents run in a circle, '
+      'stands as a project of its own, none lost', () {
+    final tree = projectTree([
+      project('Orphan', parent: 'deleted'),
+      project('A', parent: 'B'),
+      project('B', parent: 'A'),
+    ]);
+    expect([for (final node in tree) node.project.id], ['Orphan', 'A', 'B']);
+    expect(tree.first.depth, 0);
+  });
+
+  test('a project is offered as a parent but for itself and its '
+      'sub-projects at every depth', () {
+    expect(descendantsOf(projects, 'Home'), {'Garden', 'Beds', 'Kitchen'});
+    expect(
+      [for (final choice in parentChoices(projects, 'Garden')) choice.id],
+      ['Home', 'Kitchen', 'Work'],
+    );
+    expect(
+      [for (final choice in parentChoices(projects, 'Home')) choice.id],
+      ['Work'],
+    );
+    expect(
+      [for (final choice in parentChoices(projects, null)) choice.id],
+      ['Home', 'Garden', 'Beds', 'Kitchen', 'Work'],
+    );
+  });
+}

docs/adr/0033-halkyon-todo.md

@@ -461,6 +461,32 @@ The developer's review of 4 Nov 2026, again beside Todoist's screens.
 - **D27 Listed in F-Droid as *Task* and *Calendar & Agenda*** (the developer's, both,
   over *Time*, which F-Droid's categories give to clocks and timers): no other app of
   the repository used *Time*, so the repository no longer lists it.
+- **D28 A project's editor as Todoist's Add project, with sub-projects and
+  favourites** (the developer's, over the dialog of a name and a colour): a screen of its
+  own on a phone (*Add project*, *Edit project*, ← and ✓ in its app bar), the same in a
+  dialog where sheets are dialogs; the name counted to 120, a description, then rounded
+  rows: the colour (its palette name, opening the picker), the parent project (*No
+  parent* or a project, from a sheet listing them indented; never the project itself
+  nor any project below it, `parentChoices`, so none becomes its own ancestor), a
+  Favourite switch; then the Layout, Display's pictures (`LayoutPicker`). All of it is
+  **in the project's sealed payload** (`description`, `parentId`, `favorite`,
+  `defaultView`), no server model changed, no migration, the schema still 1; the payload
+  now keeps the fields it doesn't know, as a task's (D18), so a later version's survive
+  an edit here (a 0.2.0 device editing a project still drops these). **The default
+  Display**: a new project starts with the layout picked, and Display's *Reset all*
+  returns to it (`defaultView`, the dot on Display marking a Display that differs from
+  it); the editor sets its layout, and a layout picked anew there shows at once.
+  Projects made before keep List as their default. **Browse** lists each project's
+  sub-projects indented under it, folding under its chevron (the fold not kept, agent's
+  choice: a choice of the device, and quick to make again), and the favourite projects
+  in a *Favourites* group above the projects. A project whose parent is gone or doesn't
+  open here, or projects whose parents run in a circle (two devices each moving one
+  under the other), stand as projects of their own, none lost (`projectTree`). **Deleting
+  a project deletes its sub-projects**, with their sections and tasks, as Todoist does,
+  the confirmation naming them first (`TodoPerson.deleteProject`). *Alternatives:* the
+  sub-projects moved up a level (kept, but surprising after a confirmation that named
+  only the parent); a depth limit as Todoist's (four levels), left until deep trees turn
+  out to be a problem.
 
 ## Steps