msglist: Let recipient header overlap date, just like a message

Fixes: #479

Author: gnprice

lib/model/message_list.dart

@@ -191,16 +191,19 @@ mixin _MessageSequence {
     if (index == 0 || !haveSameRecipient(messages[index - 1], message)) {
       items.add(MessageListRecipientHeaderItem(message));
       canShareSender = false;
-    } else if (!messagesSameDay(messages[index - 1], message)) {
-      items.add(MessageListDateSeparatorItem(message));
-      canShareSender = false;
     } else {
       assert(items.last is MessageListMessageItem);
       final prevMessageItem = items.last as MessageListMessageItem;
       assert(identical(prevMessageItem.message, messages[index - 1]));
       assert(prevMessageItem.isLastInBlock);
       prevMessageItem.isLastInBlock = false;
-      canShareSender = (prevMessageItem.message.senderId == message.senderId);
+
+      if (!messagesSameDay(prevMessageItem.message, message)) {
+        items.add(MessageListDateSeparatorItem(message));
+        canShareSender = false;
+      } else {
+        canShareSender = (prevMessageItem.message.senderId == message.senderId);
+      }
     }
     items.add(MessageListMessageItem(message, content,
       showSender: !canShareSender, isLastInBlock: true));

test/model/message_list_test.dart

@@ -967,7 +967,13 @@ void checkInvariants(MessageListView model) {
       ..showSender.equals(
         forcedShowSender || model.messages[j].senderId != model.messages[j-1].senderId)
       ..isLastInBlock.equals(
-        i == model.items.length || model.items[i] is! MessageListMessageItem);
+        i == model.items.length || switch (model.items[i]) {
+          MessageListMessageItem()
+          || MessageListDateSeparatorItem() => false,
+          MessageListRecipientHeaderItem()
+          || MessageListHistoryStartItem()
+          || MessageListLoadingItem()       => true,
+        });
   }
   check(model.items).length.equals(i);
 }