content/msglist: Fix vertical position of horizontal scrollbar in codeblock

Discussion of this solution:
  #736 (comment)

Fixes: #736

Author: chrisbobbe

lib/widgets/message_list.dart

@@ -301,7 +301,7 @@ class _MessageListState extends State<MessageList> with PerAccountStoreAwareStat
     final length = model!.items.length;
     const centerSliverKey = ValueKey('center sliver');
 
-    final sliver = SliverStickyHeaderList(
+    Widget sliver = SliverStickyHeaderList(
       headerPlacement: HeaderPlacement.scrollingStart,
       delegate: SliverChildBuilderDelegate(
         // To preserve state across rebuilds for individual [MessageItem]
@@ -337,6 +337,12 @@ class _MessageListState extends State<MessageList> with PerAccountStoreAwareStat
           return _buildItem(data, i);
         }));
 
+    if (widget.narrow is CombinedFeedNarrow) {
+      // TODO(#311) If we have a bottom nav, it will pad the bottom
+      //   inset, and this shouldn't be necessary
+      sliver = SliverSafeArea(sliver: sliver);
+    }
+
     return CustomScrollView(
       // TODO: Offer `ScrollViewKeyboardDismissBehavior.interactive` (or
       //   similar) if that is ever offered:

test/widgets/message_list_test.dart

@@ -86,6 +86,22 @@ void main() {
     return scrollView.controller;
   }
 
+  group('presents message content appropriately', () {
+    // regression test for https://github.com/zulip/zulip-flutter/issues/736
+    testWidgets('content in "Combined feed" not asked to consume insets (including bottom)', (tester) async {
+      const fakePadding = FakeViewPadding(left: 10, top: 10, right: 10, bottom: 10);
+      tester.view.viewInsets = fakePadding;
+      tester.view.padding = fakePadding;
+
+      await setupMessageListPage(tester, narrow: const CombinedFeedNarrow(),
+        messages: [eg.streamMessage(content: ContentExample.codeBlockPlain.html)]);
+
+      final element = tester.element(find.byType(CodeBlock));
+      final padding = MediaQuery.of(element).padding;
+      check(padding).equals(EdgeInsets.zero);
+    });
+  });
+
   group('fetch older messages on scroll', () {
     int? itemCount(WidgetTester tester) =>
       tester.widget<CustomScrollView>(find.byType(CustomScrollView)).semanticChildCount;