msglist: Reserve space for "mark as read" button so messages don't shift up/down

This commit addresses the issue of the message list shifting downwards
when the "mark as read" button is pressed. The previous implementation
would return an empty widget when the button was pressed and cause the
message list to shift down. Now it takes a widget which will display
the button based on the opacity level and will disable the region the
button holds once the button is pressed, thus won't cause the message
list to shift down.

Fixes: #562

Author: abelaba

lib/widgets/message_list.dart

@@ -461,34 +461,39 @@ class MarkAsReadWidget extends StatelessWidget {
     final zulipLocalizations = ZulipLocalizations.of(context);
     final store = PerAccountStoreWidget.of(context);
     final unreadCount = store.unreads.countInNarrow(narrow);
-    return AnimatedCrossFade(
-      duration: const Duration(milliseconds: 300),
-      crossFadeState: (unreadCount > 0) ? CrossFadeState.showSecond : CrossFadeState.showFirst,
-      firstChild: const SizedBox.shrink(),
-      secondChild: SizedBox(width: double.infinity,
-        // Design referenced from:
-        //   https://www.figma.com/file/1JTNtYo9memgW7vV6d0ygq/Zulip-Mobile?type=design&node-id=132-9684&mode=design&t=jJwHzloKJ0TMOG4M-0
-        child: Padding(
-          // vertical padding adjusted for tap target height (48px) of button
-          padding: const EdgeInsets.symmetric(horizontal: 10, vertical: 10 - ((48 - 38) / 2)),
-          child: FilledButton.icon(
-            style: FilledButton.styleFrom(
-              backgroundColor: _UnreadMarker.color,
-              minimumSize: const Size.fromHeight(38),
-              textStyle:
-                // Restate [FilledButton]'s default, which inherits from
-                // [zulipTypography]…
-                Theme.of(context).textTheme.labelLarge!
-                // …then clobber some attributes to follow Figma:
-                .merge(const TextStyle(
-                  fontSize: 18,
-                  height: (23 / 18))
-                .merge(weightVariableTextStyle(context, wght: 400))),
-              shape: RoundedRectangleBorder(borderRadius: BorderRadius.circular(7)),
-            ),
-            onPressed: () => _handlePress(context),
-            icon: const Icon(Icons.playlist_add_check),
-            label: Text(zulipLocalizations.markAllAsReadLabel)))));
+    final areMessagesRead = unreadCount == 0;
+
+    return IgnorePointer(
+      ignoring: areMessagesRead,
+      child: AnimatedOpacity(
+        opacity: areMessagesRead ? 0 : 1,
+        duration: Duration(milliseconds: areMessagesRead ? 2000 : 300),
+        curve: Curves.easeOut,
+        child: SizedBox(width: double.infinity,
+          // Design referenced from:
+          //   https://www.figma.com/file/1JTNtYo9memgW7vV6d0ygq/Zulip-Mobile?type=design&node-id=132-9684&mode=design&t=jJwHzloKJ0TMOG4M-0
+          child: Padding(
+            // vertical padding adjusted for tap target height (48px) of button
+            padding: const EdgeInsets.symmetric(horizontal: 10, vertical: 10 - ((48 - 38) / 2)),
+            child: FilledButton.icon(
+              style: FilledButton.styleFrom(
+                backgroundColor: _UnreadMarker.color,
+                minimumSize: const Size.fromHeight(38),
+                textStyle:
+                  // Restate [FilledButton]'s default, which inherits from
+                  // [zulipTypography]…
+                  Theme.of(context).textTheme.labelLarge!
+                  // …then clobber some attributes to follow Figma:
+                  .merge(const TextStyle(
+                    fontSize: 18,
+                    height: (23 / 18))
+                  .merge(weightVariableTextStyle(context, wght: 400))),
+                shape: RoundedRectangleBorder(borderRadius: BorderRadius.circular(7)),
+              ),
+              onPressed: () => _handlePress(context),
+              icon: const Icon(Icons.playlist_add_check),
+              label: Text(zulipLocalizations.markAllAsReadLabel))))),
+    );
   }
 }
 

test/widgets/message_list_test.dart

@@ -610,13 +610,10 @@ void main() {
 
   group('MarkAsReadWidget', () {
     bool isMarkAsReadButtonVisible(WidgetTester tester) {
-      // Zero height elements on the edge of a scrolling viewport
-      // are treated as invisible for hit-testing, see
-      // [SliverMultiBoxAdaptorElement.debugVisitOnstageChildren].
-      // Set `skipOffstage: false` here to safely target the
-      // [MarkAsReadWidget] even when it is inactive.
-      return tester.getSize(
-        find.byType(MarkAsReadWidget, skipOffstage: false)).height > 0;
+      final zulipLocalizations = GlobalLocalizations.zulipLocalizations;
+      final finder = find.text(
+        zulipLocalizations.markAllAsReadLabel).hitTestable();
+      return finder.evaluate().isNotEmpty;
     }
 
     testWidgets('from read to unread', (WidgetTester tester) async {
@@ -648,6 +645,31 @@ void main() {
       check(isMarkAsReadButtonVisible(tester)).isFalse();
     });
 
+    testWidgets("messages don't shift position", (WidgetTester tester) async {
+      final message = eg.streamMessage(flags: []);
+      final unreadMsgs = eg.unreadMsgs(streams:[
+        UnreadStreamSnapshot(topic: message.subject, streamId: message.streamId,
+          unreadMessageIds: [message.id])
+      ]);
+      await setupMessageListPage(tester,
+        messages: [message], unreadMsgs: unreadMsgs);
+      check(isMarkAsReadButtonVisible(tester)).isTrue();
+      check(tester.widgetList(find.byType(MessageItem))).length.equals(1);
+      final before = tester.getTopLeft(find.byType(MessageItem)).dy;
+
+      store.handleEvent(UpdateMessageFlagsAddEvent(
+        id: 1,
+        flag: MessageFlag.read,
+        messages: [message.id],
+        all: false,
+      ));
+      await tester.pumpAndSettle();
+      check(isMarkAsReadButtonVisible(tester)).isFalse();
+      check(tester.widgetList(find.byType(MessageItem))).length.equals(1);
+      final after = tester.getTopLeft(find.byType(MessageItem)).dy;
+      check(after).equals(before);
+    });
+
     group('onPressed behavior', () {
       final message = eg.streamMessage(flags: []);
       final unreadMsgs = eg.unreadMsgs(streams: [