wip msglist: Jump, not scroll, to end when it might be far; TODO test

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 +756,20 @@ class MessageListView with ChangeNotifier, _MessageSequence {
     }
   }
 
+  /// Reset this view to start from the newest messages.
+  ///
+  /// This will set [anchor] to [AnchorCode.newest],
+  /// and cause messages to be re-fetched from scratch.
+  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

@@ -552,6 +552,7 @@ 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.
@@ -627,6 +628,7 @@ class _MessageListState extends State<MessageList> with PerAccountStoreAwareStat
                   //   MessageList's dartdoc.
                   child: SafeArea(
                     child: ScrollToBottomButton(
+                      model: model,
                       scrollController: scrollController,
                       visible: _scrollToBottomVisible))),
               ])))));
@@ -826,13 +828,40 @@ 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) {
+      // Scrolling smoothly from here to the bottom won't require any requests
+      // to the server.
+      // It also probably isn't *that* far away: the user must have scrolled
+      // here from there (or from near enough that a fetch reached there),
+      // so scrolling back there -- at top speed -- shouldn't take too long.
+      // Go for it.
+      scrollController.position.scrollToEnd();
+    } else {
+      // This message list doesn't have the messages for the bottom of history.
+      // There could be quite a lot of history between here and there --
+      // 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.
+      // In that case trying to scroll smoothly to the bottom is hopeless.
+      //
+      // Given that there were at least 100 messages between this message list's
+      // initial anchor and the end of history (or else `fetchInitial` would
+      // have reached the end at the outset), that situation is very likely.
+      // Even if the end is close by, it's at least one fetch away.
+      // Instead of scrolling, jump to the end, which is always just one fetch.
+      model.jumpToEnd();
+    }
   }
 
   @override