sticky_header: Rewrite sticky headers completely; write tests

lib/widgets/message_list.dart

@@ -280,11 +280,10 @@ class _MessageListState extends State<MessageList> with PerAccountStoreAwareStat
               child: Padding(
                 padding: EdgeInsets.symmetric(vertical: 16.0),
                 child: CircularProgressIndicator())), // TODO perhaps a different indicator
-          MessageListMessageItem(:var message, :var content) =>
+          MessageListMessageItem() =>
             MessageItem(
               trailing: i == 0 ? const SizedBox(height: 8) : const SizedBox(height: 11),
-              message: message,
-              content: content)
+              item: data),
         };
       });
   }
@@ -323,39 +322,51 @@ class ScrollToBottomButton extends StatelessWidget {
   }
 }
 
+class RecipientHeader extends StatelessWidget {
+  const RecipientHeader({super.key, required this.message});
+
+  final Message message;
+
+  @override
+  Widget build(BuildContext context) {
+    // TODO recipient headings depend on narrow
+    final message = this.message;
+    switch (message) {
+      case StreamMessage():
+        final store = PerAccountStoreWidget.of(context);
+        final subscription = store.subscriptions[message.streamId];
+        return StreamTopicRecipientHeader(
+          message: message, streamColor: colorForStream(subscription));
+      case DmMessage():
+        return DmRecipientHeader(message: message);
+    }
+  }
+}
+
 class MessageItem extends StatelessWidget {
   const MessageItem({
     super.key,
-    required this.message,
-    required this.content,
+    required this.item,
     this.trailing,
   });
 
-  final Message message;
-  final ZulipContent content;
+  final MessageListMessageItem item;
   final Widget? trailing;
 
   @override
   Widget build(BuildContext context) {
-    // TODO recipient headings depend on narrow
-
     final store = PerAccountStoreWidget.of(context);
+    final message = item.message;
 
     Color highlightBorderColor;
     Color restBorderColor;
-    Widget recipientHeader;
     if (message is StreamMessage) {
-      final msg = (message as StreamMessage);
-      final subscription = store.subscriptions[msg.streamId];
+      final subscription = store.subscriptions[message.streamId];
       highlightBorderColor = colorForStream(subscription);
       restBorderColor = _kStreamMessageBorderColor;
-      recipientHeader = StreamTopicRecipientHeader(
-        message: msg, streamColor: highlightBorderColor);
     } else if (message is DmMessage) {
-      final msg = (message as DmMessage);
       highlightBorderColor = _kDmRecipientHeaderColor;
       restBorderColor = _kDmRecipientHeaderColor;
-      recipientHeader = DmRecipientHeader(message: msg);
     } else {
       throw Exception("impossible message type: ${message.runtimeType}");
     }
@@ -371,12 +382,14 @@ class MessageItem extends StatelessWidget {
       shape: Border(
         left: recipientBorder, bottom: restBorder, right: restBorder));
 
-    return StickyHeader(
+    final recipientHeader = RecipientHeader(message: message);
+    return StickyHeaderItem(
       header: recipientHeader,
-      content: Column(children: [
+      child: Column(children: [
+        recipientHeader,
         DecoratedBox(
           decoration: borderDecoration,
-          child: MessageWithSender(message: message, content: content)),
+          child: MessageWithSender(message: message, content: item.content)),
         if (trailing != null) trailing!,
       ]));