msglist: Always place the sliver boundary at a message

This brings the behavior closer to what we'll want in the future,
and so allows the next commit affecting `middleItem` to be NFC.

This itself is practically NFC.  It's entirely NFC except in the
case where there are no messages, so that `items` consists only of a
history-start marker; and in that case, it causes that marker to go
in the top sliver rather than the bottom.

I believe the only observable effect of that change is that if the
viewport is so very short that the history-start marker is too tall
to fit (so that it's possible to scroll when that's the only item),
then the initial scroll position will be different.

Author: gnprice

lib/model/message_list.dart

@@ -161,7 +161,28 @@ mixin _MessageSequence {
   ///
   /// 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;
+  ///
+  /// If the bottom slice is not empty,
+  /// then its first item is a [MessageListMessageItem].
+  int get middleItem {
+    switch (items.lastOrNull) {
+      case null:
+        return 0;
+
+      case MessageListHistoryStartItem():
+        assert(items.length == 1);
+        return items.length;
+
+      case MessageListMessageItem():
+        return items.length - 1;
+
+      case MessageListLoadingItem(direction: MessageListDirection.older):
+      case MessageListRecipientHeaderItem():
+      case MessageListDateSeparatorItem():
+        assert(false, "unexpected type of last item");
+        return items.length - 1;
+    }
+  }
 
   int _findMessageWithId(int messageId) {
     return binarySearchByKey(messages, messageId,

test/model/message_list_test.dart

@@ -2010,6 +2010,9 @@ void checkInvariants(MessageListView model) {
   check(model).middleItem
     ..isGreaterOrEqual(0)
     ..isLessOrEqual(model.items.length);
+  if (model.middleItem < model.items.length) {
+    check(model.items[model.middleItem]).isA<MessageListMessageItem>();
+  }
 }
 
 extension MessageListRecipientHeaderItemChecks on Subject<MessageListRecipientHeaderItem> {