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

lib/widgets/inbox.dart

@@ -209,11 +209,19 @@ abstract class _HeaderItem extends StatelessWidget {
   final int count;
   final bool hasMention;
 
+  /// Context to access 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 +231,14 @@ 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 +286,7 @@ 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 +296,8 @@ 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 +321,7 @@ class _AllDmsSection extends StatelessWidget {
       hasMention: data.hasMention,
       collapsed: collapsed,
       pageState: pageState,
+      sectionContext: context,
     );
     return StickyHeaderItem(
       header: header,
@@ -386,6 +404,7 @@ class _StreamHeaderItem extends _HeaderItem {
     required super.pageState,
     required super.count,
     required super.hasMention,
+    required super.sectionContext,
   });
 
   @override get title => subscription.name;
@@ -397,7 +416,8 @@ 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 +447,7 @@ class _StreamSection extends StatelessWidget {
       hasMention: data.hasMention,
       collapsed: collapsed,
       pageState: pageState,
+      sectionContext: context,
     );
     return StickyHeaderItem(
       header: header,

test/widgets/inbox_test.dart

@@ -51,6 +51,11 @@ void main() {
     await tester.pumpAndSettle();
   }
 
+  List<StreamMessage> generateStreamMessages(ZulipStream stream, int count) {
+    return List.generate(count, (index) => eg.streamMessage(
+      stream: stream, topic: '${stream.name} topic $index', 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');
@@ -64,7 +69,9 @@ void main() {
       users: [eg.selfUser, eg.otherUser, eg.thirdUser],
       unreadMessages: [
         eg.streamMessage(stream: stream1, topic: 'specific topic', flags: []),
+        ...generateStreamMessages(stream1, 10),
         eg.streamMessage(stream: stream2, flags: []),
+        ...generateStreamMessages(stream2, 40),
         eg.dmMessage(from: eg.otherUser, to: [eg.selfUser], flags: []),
         eg.dmMessage(from: eg.otherUser, to: [eg.selfUser, eg.thirdUser], flags: []),
       ]);
@@ -310,6 +317,30 @@ void main() {
           checkAppearsUncollapsed(tester, findSectionContent);
         });
 
+        testWidgets('collapse all-DMs section after scroll', (tester) async {
+          await setupVarious(tester);
+
+           final listFinder = find.byType(Scrollable);
+          // Scroll the [StickyHeaderListView] enough so that
+          // the [_AllDmsSection] shows a sticky header
+          await tester.drag(listFinder, const Offset(0, -50));
+
+          final headerRow = findAllDmsHeaderRow(tester);
+          // Check that the sticky header is present
+          check(headerRow).isNotNull();
+
+          final findSectionContent = find.text(eg.otherUser.fullName);
+
+          checkAppearsUncollapsed(tester, findSectionContent);
+          await tapCollapseIcon(tester);
+          // Check that the header is still visible even after
+          // collapsing the section
+          check(headerRow).isNotNull();
+          checkAppearsCollapsed(tester, findSectionContent);
+          await tapCollapseIcon(tester);
+          checkAppearsUncollapsed(tester, findSectionContent);
+        });
+
         // 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 +416,30 @@ void main() {
           checkAppearsUncollapsed(tester, 1, findSectionContent);
         });
 
+        testWidgets('collapse stream section after scroll', (tester) async {
+          await setupVarious(tester);
+
+          final listFinder = find.byType(Scrollable);
+          // Scroll the [StickyHeaderListView] enough so that
+          // the [_StreamSection] shows a sticky header
+          await tester.drag(listFinder, const Offset(0, -200));
+
+          final headerRow = findStreamHeaderRow(tester, 1);
+          // Check that the sticky header is present
+          check(headerRow).isNotNull();
+
+          final findSectionContent = find.text('specific topic');
+
+          checkAppearsUncollapsed(tester, 1, findSectionContent);
+          await tapCollapseIcon(tester, 1);
+          // Check that the header is still visible even after
+          // collapsing the section
+          check(headerRow).isNotNull();
+          checkAppearsCollapsed(tester, 1, findSectionContent);
+          await tapCollapseIcon(tester, 1);
+          checkAppearsUncollapsed(tester, 1, findSectionContent);
+        });
+
         // 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