msglist [nfc]: Define sliver boundary by messages

This gives a bit more structured of an idea of what `middleItem`
is supposed to mean.  We'll use this for maintaining `middleItem`
as a more dynamic value in upcoming commits.

Author: gnprice

lib/model/message_list.dart

@@ -91,9 +91,20 @@ mixin _MessageSequence {
   /// conceptually belongs in this message list.
   /// That information is expressed in [fetched] and [haveOldest].
   ///
+  /// See also [middleMessage], an index which divides this list
+  /// into a top slice and a bottom slice.
+  ///
   /// See also [contents] and [items].
   final List<Message> messages = [];
 
+  /// An index into [messages] dividing it into a top slice and a bottom slice.
+  ///
+  /// The indices 0 to before [middleMessage] are the top slice of [messages],
+  /// and the indices from [middleMessage] to the end are the bottom slice.
+  ///
+  /// The corresponding item index is [middleItem].
+  int get middleMessage => messages.isEmpty ? 0 : messages.length - 1;
+
   /// Whether [messages] and [items] represent the results of a fetch.
   ///
   /// This allows the UI to distinguish "still working on fetching messages"
@@ -162,8 +173,11 @@ 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.
   ///
-  /// If the bottom slice is not empty,
-  /// then its first item is a [MessageListMessageItem].
+  /// The top and bottom slices of [items] correspond to
+  /// the top and bottom slices of [messages] respectively.
+  /// Either the bottom slices of both [items] and [messages] are empty,
+  /// or the first item in the bottom slice of [items] is a [MessageListMessageItem]
+  /// for the first message in the bottom slice of [messages].
   int get middleItem {
     switch (items.lastOrNull) {
       case null:

test/model/message_list_test.dart

@@ -1961,6 +1961,10 @@ void checkInvariants(MessageListView model) {
   check(isSortedWithoutDuplicates(model.messages.map((m) => m.id).toList()))
     .isTrue();
 
+  check(model).middleMessage
+    ..isGreaterOrEqual(0)
+    ..isLessOrEqual(model.messages.length);
+
   check(model).contents.length.equals(model.messages.length);
   for (int i = 0; i < model.contents.length; i++) {
     final poll = model.messages[i].poll;
@@ -2010,8 +2014,11 @@ 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>();
+  if (model.middleItem == model.items.length) {
+    check(model.middleMessage).equals(model.messages.length);
+  } else {
+    check(model.items[model.middleItem]).isA<MessageListMessageItem>()
+      .message.identicalTo(model.messages[model.middleMessage]);
   }
 }
 
@@ -2038,6 +2045,7 @@ extension MessageListViewChecks on Subject<MessageListView> {
   Subject<PerAccountStore> get store => has((x) => x.store, 'store');
   Subject<Narrow> get narrow => has((x) => x.narrow, 'narrow');
   Subject<List<Message>> get messages => has((x) => x.messages, 'messages');
+  Subject<int> get middleMessage => has((x) => x.middleMessage, 'middleMessage');
   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');