wip jumpToEnd; TODO test, doc; TODO discuss criterion a bit more

When the message list is truly far back in history -- for example,
at first unread in the combined feed or a busy channel, for a user
who has some old unreads going back months and years -- trying to
scroll smoothly to the bottom is hopeless.  The only way to get to
the newest messages in any reasonable amount of time is to jump there.

So, do that.

Author: gnprice

lib/model/message_list.dart

@@ -479,7 +479,7 @@ class MessageListView with ChangeNotifier, _MessageSequence {
   /// which might be made internally by this class in order to
   /// fetch the messages from scratch, e.g. after certain events.
   Anchor get anchor => _anchor;
-  final Anchor _anchor;
+  Anchor _anchor;
 
   void _register() {
     store.registerMessageList(this);
@@ -756,6 +756,16 @@ class MessageListView with ChangeNotifier, _MessageSequence {
     }
   }
 
+  void jumpToEnd() {
+    assert(fetched);
+    assert(!haveNewest);
+    assert(anchor != AnchorCode.newest);
+    _anchor = AnchorCode.newest;
+    _reset();
+    notifyListeners();
+    fetchInitial();
+  }
+
   /// Add [outboxMessage] if it belongs to the view.
   void addOutboxMessage(OutboxMessage outboxMessage) {
     // TODO(#1441) implement this

lib/widgets/message_list.dart

@@ -543,6 +543,7 @@ class _MessageListState extends State<MessageList> with PerAccountStoreAwareStat
       //   redirected us to the new location of the operand message ID.
       widget.onNarrowChanged(model.narrow);
     }
+    // TODO when model reset, reset scroll
     setState(() {
       // The actual state lives in the [MessageListView] model.
       // This method was called because that just changed.
@@ -618,6 +619,7 @@ class _MessageListState extends State<MessageList> with PerAccountStoreAwareStat
                   //   MessageList's dartdoc.
                   child: SafeArea(
                     child: ScrollToBottomButton(
+                      model: model,
                       scrollController: scrollController,
                       visible: _scrollToBottomVisible))),
               ])))));
@@ -817,13 +819,23 @@ class _MessageListLoadingMore extends StatelessWidget {
 }
 
 class ScrollToBottomButton extends StatelessWidget {
-  const ScrollToBottomButton({super.key, required this.scrollController, required this.visible});
+  const ScrollToBottomButton({
+    super.key,
+    required this.model,
+    required this.scrollController,
+    required this.visible,
+  });
 
-  final ValueNotifier<bool> visible;
+  final MessageListView model;
   final MessageListScrollController scrollController;
+  final ValueNotifier<bool> visible;
 
   void _scrollToBottom() {
-    scrollController.position.scrollToEnd();
+    if (model.haveNewest) {
+      scrollController.position.scrollToEnd();
+    } else {
+      model.jumpToEnd();
+    }
   }
 
   @override