msglist [nfc]: Let ScrollView count forward, and sliver handle reverse

This means that the overall scroll view has AxisDirection.down
as the scroll direction, rather than AxisDirection.up, and the
SliverStickyHeader is passed GrowthDirection.reverse instead
of GrowthDirection.forward in order to get the same effect.

This makes the scrolling a little easier to think about when focused
on MessageList's code, because for example (as seen in this diff)
`scrollMetrics.extentBefore` now refers to the messages that are
older than the ones on screen, rather than those that are newer.

This also brings us closer to the setup we'll want for #80 and #82,
opening the message list at an anchor other than the end: the empty
placeholder sliver at the bottom will become the list of messages
after the starting anchor, while the sliver at the top will remain
the list of messages above the anchor.

Author: gnprice

Diff for: lib/widgets/message_list.dart

@@ -218,7 +218,7 @@ class _MessageListState extends State<MessageList> with PerAccountStoreAwareStat
       _scrollToBottomVisibleValue.value = true;
     }
 
-    if (scrollMetrics.extentAfter < kFetchMessagesBufferPixels) {
+    if (scrollMetrics.extentBefore < kFetchMessagesBufferPixels) {
       // TODO: This ends up firing a second time shortly after we fetch a batch.
       //   The result is that each time we decide to fetch a batch, we end up
       //   fetching two batches in quick succession.  This is basically harmless
@@ -278,6 +278,7 @@ class _MessageListState extends State<MessageList> with PerAccountStoreAwareStat
 
   Widget _buildListView(context) {
     final length = model!.items.length;
+    const centerSliverKey = ValueKey('center sliver');
     return CustomScrollView(
       // TODO: Offer `ScrollViewKeyboardDismissBehavior.interactive` (or
       //   similar) if that is ever offered:
@@ -291,18 +292,12 @@ class _MessageListState extends State<MessageList> with PerAccountStoreAwareStat
       },
 
       controller: scrollController,
-
-      // Setting reverse: true means the scroll starts at the bottom.
-      // Flipping the indexes (in the SliverChildBuilderDelegate callback)
-      // means the start/bottom has the latest messages.
-      // This works great when we want to start from the latest.
-      // TODO handle scroll starting at first unread, or link anchor
-      // TODO on new message when scrolled up, anchor scroll to what's in view
-      reverse: true,
+      anchor: 1.0,
+      center: centerSliverKey,
 
       slivers: [
         SliverStickyHeaderList(
-          headerPlacement: HeaderPlacement.scrollingEnd,
+          headerPlacement: HeaderPlacement.scrollingStart,
           delegate: SliverChildBuilderDelegate(
             // To preserve state across rebuilds for individual [MessageItem]
             // widgets as the size of [MessageListView.items] changes we need
@@ -336,6 +331,11 @@ class _MessageListState extends State<MessageList> with PerAccountStoreAwareStat
               final data = model!.items[length - 1 - (i - 2)];
               return _buildItem(data, i);
             })),
+
+        // 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),
       ]);
   }