msglist [nfc]: Move sliver boundary into the view-model

This will allow the model to maintain it over time as newer messages
arrive or get fetched.

Author: gnprice

lib/model/message_list.dart

@@ -152,8 +152,17 @@ mixin _MessageSequence {
   /// This information is completely derived from [messages] and
   /// the flags [haveOldest], [fetchingOlder] and [fetchOlderCoolingDown].
   /// It exists as an optimization, to memoize that computation.
+  ///
+  /// See also [middleItem], an index which divides this list
+  /// into a top slice and a bottom slice.
   final QueueList<MessageListItem> items = QueueList();
 
+  /// An index into [items] dividing it into a top slice and a bottom slice.
+  ///
+  /// The indices 0 to before [middleItem] are the top slice of [items],
+  /// and the indices from [middleItem] to the end are the bottom slice.
+  int get middleItem => items.isEmpty ? 0 : items.length - 1;
+
   int _findMessageWithId(int messageId) {
     return binarySearchByKey(messages, messageId,
       (message, messageId) => message.id.compareTo(messageId));

lib/widgets/message_list.dart

@@ -572,10 +572,9 @@ class _MessageListState extends State<MessageList> with PerAccountStoreAwareStat
     // The list has two slivers: a top sliver growing upward,
     // and a bottom sliver growing downward.
     // Each sliver has some of the items from `model!.items`.
-    const maxBottomItems = 1;
     final totalItems = model!.items.length;
-    final bottomItems = totalItems <= maxBottomItems ? totalItems : maxBottomItems;
-    final topItems = totalItems - bottomItems;
+    final topItems = model!.middleItem;
+    final bottomItems = totalItems - topItems;
 
     // The top sliver has its child 0 as the item just before the
     // sliver boundary, child 1 as the item before that, and so on.

test/model/message_list_test.dart

@@ -2006,6 +2006,10 @@ void checkInvariants(MessageListView model) {
         });
   }
   check(model.items).length.equals(i);
+
+  check(model).middleItem
+    ..isGreaterOrEqual(0)
+    ..isLessOrEqual(model.items.length);
 }
 
 extension MessageListRecipientHeaderItemChecks on Subject<MessageListRecipientHeaderItem> {
@@ -2033,6 +2037,7 @@ extension MessageListViewChecks on Subject<MessageListView> {
   Subject<List<Message>> get messages => has((x) => x.messages, 'messages');
   Subject<List<ZulipMessageContent>> get contents => has((x) => x.contents, 'contents');
   Subject<List<MessageListItem>> get items => has((x) => x.items, 'items');
+  Subject<int> get middleItem => has((x) => x.middleItem, 'middleItem');
   Subject<bool> get fetched => has((x) => x.fetched, 'fetched');
   Subject<bool> get haveOldest => has((x) => x.haveOldest, 'haveOldest');
   Subject<bool> get fetchingOlder => has((x) => x.fetchingOlder, 'fetchingOlder');