msglist: Opt out of adding AutomaticKeepAlive widgets to the tree

Without these, it'll be more efficient to walk the element tree to
find which messages are in the viewport. For mark-read-on-scroll,
coming up.

Author: chrisbobbe

lib/widgets/message_list.dart

@@ -666,6 +666,10 @@ class _MessageListState extends State<MessageList> with PerAccountStoreAwareStat
     final topSliver = SliverStickyHeaderList(
       headerPlacement: HeaderPlacement.scrollingStart,
       delegate: SliverChildBuilderDelegate(
+        // None of the children ever need to keep themselves alive;
+        // opt out of adding AutomaticKeepAlive widgets that we don't need.
+        addAutomaticKeepAlives: false,
+
         // 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
@@ -706,6 +710,10 @@ class _MessageListState extends State<MessageList> with PerAccountStoreAwareStat
       key: centerSliverKey,
       headerPlacement: HeaderPlacement.scrollingStart,
       delegate: SliverChildBuilderDelegate(
+        // None of the children ever need to keep themselves alive;
+        // opt out of adding AutomaticKeepAlive widgets that we don't need.
+        addAutomaticKeepAlives: false,
+
         // 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