msglist [nfc]: Pull out sliver helper for our SliverStickyHeaderList

Author: chrisbobbe

lib/widgets/message_list.dart

@@ -298,6 +298,43 @@ class _MessageListState extends State<MessageList> with PerAccountStoreAwareStat
   Widget _buildListView(BuildContext context) {
     final length = model!.items.length;
     const centerSliverKey = ValueKey('center sliver');
+
+    final sliver = SliverStickyHeaderList(
+      headerPlacement: HeaderPlacement.scrollingStart,
+      delegate: SliverChildBuilderDelegate(
+        // To preserve state across rebuilds for individual [MessageItem]
+        // widgets as the size of [MessageListView.items] changes we need
+        // to match old widgets by their key to their new position in
+        // the list.
+        //
+        // The keys are of type [ValueKey] with a value of [Message.id]
+        // and here we use a O(log n) binary search method. This could
+        // be improved but for now it only triggers for materialized
+        // widgets. As a simple test, flinging through Combined feed in
+        // CZO on a Pixel 5, this only runs about 10 times per rebuild
+        // and the timing for each call is <100 microseconds.
+        //
+        // Non-message items (e.g., start and end markers) that do not
+        // have state that needs to be preserved have not been given keys
+        // and will not trigger this callback.
+        findChildIndexCallback: (Key key) {
+          final valueKey = key as ValueKey<int>;
+          final index = model!.findItemWithMessageId(valueKey.value);
+          if (index == -1) return null;
+          return length - 1 - (index - 2);
+        },
+        childCount: length + 2,
+        (context, i) {
+          // To reinforce that the end of the feed has been reached:
+          //   https://chat.zulip.org/#narrow/stream/243-mobile-team/topic/flutter.3A.20Mark-as-read/near/1680603
+          if (i == 0) return const SizedBox(height: 36);
+
+          if (i == 1) return MarkAsReadWidget(narrow: widget.narrow);
+
+          final data = model!.items[length - 1 - (i - 2)];
+          return _buildItem(data, i);
+        }));
+
     return CustomScrollView(
       // TODO: Offer `ScrollViewKeyboardDismissBehavior.interactive` (or
       //   similar) if that is ever offered:
@@ -316,41 +353,7 @@ class _MessageListState extends State<MessageList> with PerAccountStoreAwareStat
       center: centerSliverKey,
 
       slivers: [
-        SliverStickyHeaderList(
-          headerPlacement: HeaderPlacement.scrollingStart,
-          delegate: SliverChildBuilderDelegate(
-            // To preserve state across rebuilds for individual [MessageItem]
-            // widgets as the size of [MessageListView.items] changes we need
-            // to match old widgets by their key to their new position in
-            // the list.
-            //
-            // The keys are of type [ValueKey] with a value of [Message.id]
-            // and here we use a O(log n) binary search method. This could
-            // be improved but for now it only triggers for materialized
-            // widgets. As a simple test, flinging through Combined feed in
-            // CZO on a Pixel 5, this only runs about 10 times per rebuild
-            // and the timing for each call is <100 microseconds.
-            //
-            // Non-message items (e.g., start and end markers) that do not
-            // have state that needs to be preserved have not been given keys
-            // and will not trigger this callback.
-            findChildIndexCallback: (Key key) {
-              final valueKey = key as ValueKey<int>;
-              final index = model!.findItemWithMessageId(valueKey.value);
-              if (index == -1) return null;
-              return length - 1 - (index - 2);
-            },
-            childCount: length + 2,
-            (context, i) {
-              // To reinforce that the end of the feed has been reached:
-              //   https://chat.zulip.org/#narrow/stream/243-mobile-team/topic/flutter.3A.20Mark-as-read/near/1680603
-              if (i == 0) return const SizedBox(height: 36);
-
-              if (i == 1) return MarkAsReadWidget(narrow: widget.narrow);
-
-              final data = model!.items[length - 1 - (i - 2)];
-              return _buildItem(data, i);
-            })),
+        sliver,
 
         // This is a trivial placeholder that occupies no space.  Its purpose is
         // to have the key that's passed to [ScrollView.center], and so to cause