WIP msglist: Split into back-to-back slivers; TODO test?; TODO scroll-to-end show, act; TODO weaken test on crossing slivers

Thanks to all the preparatory changes we've made in this commit
series and those that came before it, this change should have only
subtle effects on user-visible behavior.

In particular:

 * The sticky headers should behave exactly as they did before,
   even when the sliver boundary lies under the sticky header,
   thanks to several previous commit series.

 * On first loading a given message list, it should start out
   scrolled to the end, just as before.

 * When already scrolled to the end, the message list should stay
   there when a new message arrives, or a message is edited, etc.,
   even if the (new) latest message is taller than it was.

Subtle differences include:

 * When scrolled up from the bottom and a new message comes in,
   the behavior is slightly different from before.  The current
   exact behavior is something we probably want to change anyway,
   and I think the new behavior isn't particularly better or worse.

 * The behavior upon overscroll might be slightly different;
   I'm not sure.

This therefore serves as a testing ground for all the ways that the
message list's scrolling behavior can become more complicated when
two (nontrivial) slivers are involved.  If we find a situation
where the behavior does change noticeably, that will be something
to investigate and fix.

Author: gnprice

lib/widgets/message_list.dart

@@ -583,11 +583,18 @@ class _MessageListState extends State<MessageList> with PerAccountStoreAwareStat
   }
 
   Widget _buildListView(BuildContext context) {
+    const bottomSize = 1;
     final length = model!.items.length;
+    final bottomLength = length <= bottomSize ? length : bottomSize;
+    final topLength = length - bottomLength;
     const centerSliverKey = ValueKey('center sliver');
     final zulipLocalizations = ZulipLocalizations.of(context);
 
-    Widget sliver = SliverStickyHeaderList(
+    // TODO(#311) If we have a bottom nav, it will pad the bottom inset,
+    //   and this can be removed; also remove mention in MessageList dartdoc
+    final needSafeArea = !ComposeBox.hasComposeBox(widget.narrow);
+
+    final topSliver = SliverStickyHeaderList(
       headerPlacement: HeaderPlacement.scrollingStart,
       delegate: SliverChildBuilderDelegate(
         // To preserve state across rebuilds for individual [MessageItem]
@@ -609,26 +616,60 @@ class _MessageListState extends State<MessageList> with PerAccountStoreAwareStat
           final valueKey = key as ValueKey<int>;
           final index = model!.findItemWithMessageId(valueKey.value);
           if (index == -1) return null;
-          return length - 1 - (index - 3);
+          final i = length - 1 - (index + bottomLength);
+          if (i < 0) return null;
+          return i;
+        },
+        childCount: topLength,
+        (context, i) {
+          final data = model!.items[length - 1 - (i + bottomLength)];
+          final item = _buildItem(zulipLocalizations, data);
+          return item;
+        }));
+
+    Widget bottomSliver = SliverStickyHeaderList(
+      key: needSafeArea ? null : centerSliverKey,
+      headerPlacement: HeaderPlacement.scrollingStart,
+      delegate: SliverChildBuilderDelegate(
+        // To preserve state across rebuilds for individual [MessageItem]
+        // widgets as the size of [MessageListView.items] changes we need
+        // to match old widgets by their key to their new position in
+        // the list.
+        //
+        // The keys are of type [ValueKey] with a value of [Message.id]
+        // and here we use a O(log n) binary search method. This could
+        // be improved but for now it only triggers for materialized
+        // widgets. As a simple test, flinging through All Messages in
+        // CZO on a Pixel 5, this only runs about 10 times per rebuild
+        // and the timing for each call is <100 microseconds.
+        //
+        // Non-message items (e.g., start and end markers) that do not
+        // have state that needs to be preserved have not been given keys
+        // and will not trigger this callback.
+        findChildIndexCallback: (Key key) {
+          final valueKey = key as ValueKey<int>;
+          final index = model!.findItemWithMessageId(valueKey.value);
+          if (index == -1) return null;
+          final i = index - topLength;
+          if (i < 0) return null;
+          return i;
         },
-        childCount: length + 3,
+        childCount: bottomLength + 3,
         (context, i) {
           // To reinforce that the end of the feed has been reached:
           //   https://chat.zulip.org/#narrow/stream/243-mobile-team/topic/flutter.3A.20Mark-as-read/near/1680603
-          if (i == 0) return const SizedBox(height: 36);
+          if (i == bottomLength + 2) return const SizedBox(height: 36);
 
-          if (i == 1) return MarkAsReadWidget(narrow: widget.narrow);
+          if (i == bottomLength + 1) return MarkAsReadWidget(narrow: widget.narrow);
 
-          if (i == 2) return TypingStatusWidget(narrow: widget.narrow);
+          if (i == bottomLength) return TypingStatusWidget(narrow: widget.narrow);
 
-          final data = model!.items[length - 1 - (i - 3)];
+          final data = model!.items[topLength + i];
           return _buildItem(zulipLocalizations, data);
         }));
 
-    if (!ComposeBox.hasComposeBox(widget.narrow)) {
-      // TODO(#311) If we have a bottom nav, it will pad the bottom inset,
-      //   and this can be removed; also remove mention in MessageList dartdoc
-      sliver = SliverSafeArea(sliver: sliver);
+    if (needSafeArea) {
+      bottomSliver = SliverSafeArea(key: centerSliverKey, sliver: bottomSliver);
     }
 
     return MessageListScrollView(
@@ -649,12 +690,8 @@ class _MessageListState extends State<MessageList> with PerAccountStoreAwareStat
       paintOrder: SliverPaintOrder.firstIsTop,
 
       slivers: [
-        sliver,
-
-        // This is a trivial placeholder that occupies no space.  Its purpose is
-        // to have the key that's passed to [ScrollView.center], and so to cause
-        // the above [SliverStickyHeaderList] to run from bottom to top.
-        const SliverToBoxAdapter(key: centerSliverKey),
+        topSliver,
+        bottomSliver,
       ]);
   }