inbox: Fix collapsing a section from its sticky header

Before this fix, when scrolled down through the inbox page, collapsing a
section (either the all-DMs section or a stream section) through its
sticky header would work but cause an abnormal behavior, pushing the
current section header and some of the following sections off the screen
by the amount of space the current section occupied.

After this fix, collapsing a section through a sticky header would work
as expected, without pushing the header and the following sections off
the screen.

Fixes: #391

Author: sm-sayedi

lib/widgets/inbox.dart

@@ -209,11 +209,19 @@ abstract class _HeaderItem extends StatelessWidget {
   final int count;
   final bool hasMention;
 
+  /// A build context within the [_StreamSection] or [_AllDmsSection].
+  ///
+  /// Used to ensure the [_StreamSection] or [_AllDmsSection] that encloses the
+  /// current [_HeaderItem] is visible after being collapsed through this
+  /// [_HeaderItem].
+  final BuildContext sectionContext;
+
   const _HeaderItem({
     required this.collapsed,
     required this.pageState,
     required this.count,
     required this.hasMention,
+    required this.sectionContext,
   });
 
   String get title;
@@ -223,7 +231,15 @@ abstract class _HeaderItem extends StatelessWidget {
   Color get uncollapsedBackgroundColor;
   Color? get unreadCountBadgeBackgroundColor;
 
-  void Function() get onCollapseButtonTap;
+  Future<void> Function() get onCollapseButtonTap => () async {
+    if (!collapsed) {
+      await Scrollable.ensureVisible(
+        sectionContext,
+        alignmentPolicy: ScrollPositionAlignmentPolicy.keepVisibleAtStart,
+      );
+    }
+  };
+
   void Function() get onRowTap;
 
   @override
@@ -271,6 +287,7 @@ class _AllDmsHeaderItem extends _HeaderItem {
     required super.pageState,
     required super.count,
     required super.hasMention,
+    required super.sectionContext,
   });
 
   @override get title => 'Direct messages'; // TODO(i18n)
@@ -280,7 +297,8 @@ class _AllDmsHeaderItem extends _HeaderItem {
   @override get uncollapsedBackgroundColor => const Color(0xFFF3F0E7);
   @override get unreadCountBadgeBackgroundColor => null;
 
-  @override get onCollapseButtonTap => () {
+  @override get onCollapseButtonTap => () async {
+    await super.onCollapseButtonTap();
     pageState.allDmsCollapsed = !collapsed;
   };
   @override get onRowTap => onCollapseButtonTap; // TODO open all-DMs narrow?
@@ -304,6 +322,7 @@ class _AllDmsSection extends StatelessWidget {
       hasMention: data.hasMention,
       collapsed: collapsed,
       pageState: pageState,
+      sectionContext: context,
     );
     return StickyHeaderItem(
       header: header,
@@ -386,6 +405,7 @@ class _StreamHeaderItem extends _HeaderItem {
     required super.pageState,
     required super.count,
     required super.hasMention,
+    required super.sectionContext,
   });
 
   @override get title => subscription.name;
@@ -397,7 +417,8 @@ class _StreamHeaderItem extends _HeaderItem {
   @override get unreadCountBadgeBackgroundColor =>
     subscription.colorSwatch().unreadCountBadgeBackground;
 
-  @override get onCollapseButtonTap => () {
+  @override get onCollapseButtonTap => () async {
+    await super.onCollapseButtonTap();
     if (collapsed) {
       pageState.uncollapseStream(subscription.streamId);
     } else {
@@ -427,6 +448,7 @@ class _StreamSection extends StatelessWidget {
       hasMention: data.hasMention,
       collapsed: collapsed,
       pageState: pageState,
+      sectionContext: context,
     );
     return StickyHeaderItem(
       header: header,

test/example_data.dart

@@ -119,6 +119,8 @@ final Account otherAccount = account(
 
 final User thirdUser = user(fullName: 'Third User', email: 'third@example');
 
+final User fourthUser  = user(fullName: 'Fourth User', email: 'fourth@example');
+
 ////////////////////////////////////////////////////////////////
 // Streams and subscriptions.
 //

test/widgets/inbox_test.dart

@@ -13,6 +13,38 @@ import '../flutter_checks.dart';
 import '../model/binding.dart';
 import '../model/test_store.dart';
 
+/// Repeatedly drags `view` by `moveStep` until `finder` is invisible.
+///
+/// Between each drag, advances the clock by `duration`.
+///
+/// Throws a [StateError] if `finder` is still visible after `maxIteration`
+/// drags.
+///
+/// See also:
+///  * [WidgetController.dragUntilVisible], which does the inverse.
+Future<void> dragUntilInvisible(
+  WidgetTester tester,
+  FinderBase<Element> finder,
+  FinderBase<Element> view,
+  Offset moveStep, {
+  int maxIteration = 50,
+  Duration duration = const Duration(milliseconds: 50),
+}) {
+  return TestAsyncUtils.guard<void>(() async {
+    final iteration = maxIteration;
+    while (maxIteration > 0 && finder.evaluate().isNotEmpty) {
+      await tester.drag(view, moveStep);
+      await tester.pump(duration);
+      maxIteration -= 1;
+    }
+    if (maxIteration <= 0 && finder.evaluate().isNotEmpty) {
+      throw StateError(
+        'Finder is still visible after $iteration iterations.'
+        ' Consider increasing the number of iterations.');
+    }
+  });
+}
+
 void main() {
   TestZulipBinding.ensureInitialized();
 
@@ -51,6 +83,15 @@ void main() {
     await tester.pumpAndSettle();
   }
 
+  List<StreamMessage> generateStreamMessages({
+    required ZulipStream stream,
+    required int count,
+    required List<MessageFlag> flags,
+  }) {
+    return List.generate(count, (index) => eg.streamMessage(
+      stream: stream, topic: '${stream.name} topic $index', flags: flags));
+  }
+
   /// Set up an inbox view with lots of interesting content.
   Future<void> setupVarious(WidgetTester tester) async {
     final stream1 = eg.stream(streamId: 1, name: 'stream 1');
@@ -61,12 +102,16 @@ void main() {
     await setupPage(tester,
       streams: [stream1, stream2],
       subscriptions: [sub1, sub2],
-      users: [eg.selfUser, eg.otherUser, eg.thirdUser],
+      users: [eg.selfUser, eg.otherUser, eg.thirdUser, eg.fourthUser],
       unreadMessages: [
         eg.streamMessage(stream: stream1, topic: 'specific topic', flags: []),
+        ...generateStreamMessages(stream: stream1, count: 10, flags: []),
         eg.streamMessage(stream: stream2, flags: []),
+        ...generateStreamMessages(stream: stream2, count: 40, flags: []),
         eg.dmMessage(from: eg.otherUser, to: [eg.selfUser], flags: []),
         eg.dmMessage(from: eg.otherUser, to: [eg.selfUser, eg.thirdUser], flags: []),
+        eg.dmMessage(from: eg.thirdUser, to: [eg.selfUser], flags: []),
+        eg.dmMessage(from: eg.fourthUser, to: [eg.selfUser], flags: []),
       ]);
   }
 
@@ -310,6 +355,28 @@ void main() {
           checkAppearsUncollapsed(tester, findSectionContent);
         });
 
+        testWidgets('collapse all-DMs section when partially offscreen: '
+          'header remains sticky at top', (tester) async {
+          await setupVarious(tester);
+
+          final listFinder = find.byType(Scrollable);
+          final dmFinder = find.text(eg.otherUser.fullName).hitTestable();
+
+          // Scroll part of [_AllDmsSection] offscreen.
+          await dragUntilInvisible(
+            tester, dmFinder, listFinder, const Offset(0, -50));
+
+          // Check that the header is present (which must therefore
+          // be as a sticky header).
+          check(findAllDmsHeaderRow(tester)).isNotNull();
+
+          await tapCollapseIcon(tester);
+
+          // Check that the header is still visible even after
+          // collapsing the section.
+          check(findAllDmsHeaderRow(tester)).isNotNull();
+        });
+
         // TODO check it remains collapsed even if you scroll far away and back
 
         // TODO check that it's always uncollapsed when it appears after being
@@ -385,6 +452,52 @@ void main() {
           checkAppearsUncollapsed(tester, 1, findSectionContent);
         });
 
+        testWidgets('collapse stream section when partially offscreen: '
+          'header remains sticky at top', (tester) async {
+          await setupVarious(tester);
+
+          final topicFinder = find.text('stream 1 topic 4').hitTestable();
+          final listFinder = find.byType(Scrollable);
+
+          // Scroll part of [_StreamSection] offscreen.
+          await dragUntilInvisible(
+            tester, topicFinder, listFinder, const Offset(0, -50));
+
+          // Check that the header is present (which must therefore
+          // be as a sticky header).
+          check(findStreamHeaderRow(tester, 1)).isNotNull();
+
+          await tapCollapseIcon(tester, 1);
+
+          // Check that the header is still visible even after
+          // collapsing the section.
+          check(findStreamHeaderRow(tester, 1)).isNotNull();
+        });
+
+        testWidgets('collapse stream section in middle of screen: '
+          'header stays fixed', (tester) async {
+          await setupVarious(tester);
+
+          final headerRow = findStreamHeaderRow(tester, 1);
+          // Check that the header is present.
+          check(headerRow).isNotNull();
+
+          final rectBeforeTap = tester.getRect(find.byWidget(headerRow!));
+          final scrollableTop = tester.getRect(find.byType(Scrollable)).top;
+          // Check that the header is somewhere in the middle of the screen.
+          check(rectBeforeTap.top).isGreaterThan(scrollableTop);
+
+          await tapCollapseIcon(tester, 1);
+
+          final headerRowAfterTap = findStreamHeaderRow(tester, 1);
+          final rectAfterTap =
+            tester.getRect(find.byWidget(headerRowAfterTap!));
+
+          // Check that the position of the header before and after
+          // collapsing is the same.
+          check(rectAfterTap).equals(rectBeforeTap);
+        });
+
         // TODO check it remains collapsed even if you scroll far away and back
 
         // TODO check that it's always uncollapsed when it appears after being