timeline fixups

lib/widgets/room_chat/chat_timeline.dart

@@ -40,6 +40,16 @@ class const ChatTimeline({
     isHighlighted: scroll.highlightedEvent == eventRowId,
   );
 
+  /// Where the row with [key] now is in [rows], so its element moves with it
+  /// when rows are added before it, rather than being rebuilt by index.
+  int? indexOf(IList<int> rows, Key key) => switch (key) {
+    EventRowKey(:final rowId) => switch (rows.indexOf(rowId)) {
+      -1 => null,
+      final index => index,
+    },
+    _ => null,
+  };
+
   @override
   Widget build(BuildContext context) => LayoutBuilder(
     builder: (context, constraints) => TweenAnimationBuilder(
@@ -66,6 +76,7 @@ class const ChatTimeline({
 
             SuperSliverList.builder(
               itemCount: scroll.liveRows.length,
+              findChildIndexCallback: (key) => indexOf(scroll.liveRows, key),
               itemBuilder: (_, index) => eventRow(
                 scroll.liveRows[index],
                 index > 0
@@ -78,6 +89,8 @@ class const ChatTimeline({
             SuperSliverList.builder(
               key: scroll.centerKey,
               itemCount: scroll.historyRows.length,
+              findChildIndexCallback: (key) =>
+                  indexOf(scroll.historyRows, key),
               itemBuilder: (_, index) => eventRow(
                 scroll.historyRows[index],
                 scroll.historyRows.getOrNull(index + 1),