scroll: Show start of latest message if long, instead of end

This makes our first payoff in actual UX from having the message list
split into two back-to-back slivers!

With this change, if you open a message list and the latest message
is very tall, the list starts out scrolled so that you can see the
top of that latest message -- plus a bit of context above it (25% of
the viewport's height).  Previously the list would always start out
scrolled to the end, so you'd have to scroll up in order to read
even the one latest message from the beginning.

In addition to a small UX improvement now, this makes a preview of
behavior we'll want to have when the bottom sliver starts at the
first unread message, and may have many messages after that.
This new behavior is nice already with one message, if the message
happens to be very tall; but it'll become critical when the bottom
sliver is routinely many screenfuls tall.

Author: gnprice

lib/widgets/scrolling.dart

@@ -333,10 +333,13 @@ class MessageListScrollPosition extends ScrollPositionWithSingleContext {
 
     if (!_hasEverCompletedLayout) {
       // The list is being laid out for the first time (its first performLayout).
-      // Start out scrolled to the end.
+      // Start out scrolled down so the bottom sliver (the new messages)
+      // occupies 75% of the viewport,
+      // or at the in-range scroll position closest to that.
       // This also brings [pixels] within bounds, which
       // the initial value of 0.0 might not have been.
-      final target = maxScrollExtent;
+      final target = clampDouble(0.75 * viewportDimension,
+        minScrollExtent, maxScrollExtent);
       if (!hasPixels || pixels != target) {
         correctPixels(target);
         changed = true;

test/widgets/scrolling_test.dart

@@ -177,20 +177,58 @@ void main() {
     });
 
     testWidgets('short/long -> scrolls to ends and no farther', (tester) async {
-      // Starts out scrolled to bottom.
+      // Starts out scrolled to top (to show top of the bottom sliver).
       await prepare(tester, topHeight: 100, bottomHeight: 800);
-      check(tester.getRect(findBottom)).bottom.equals(600);
+      check(tester.getRect(findTop)).top.equals(0);
+      check(tester.getRect(findBottom)).bottom.equals(900);
 
-      // Try scrolling down (by dragging up); doesn't move.
-      await tester.drag(findBottom, Offset(0, -100));
+      // Try scrolling up (by dragging down); doesn't move.
+      await tester.drag(findBottom, Offset(0, 100));
       await tester.pump();
-      check(tester.getRect(findBottom)).bottom.equals(600);
+      check(tester.getRect(findBottom)).bottom.equals(900);
 
-      // Try scrolling up (by dragging down); moves only as far as top of list.
-      await tester.drag(findBottom, Offset(0, 400));
+      // Try scrolling down (by dragging up); moves only as far as bottom of list.
+      await tester.drag(findBottom, Offset(0, -400));
       await tester.pump();
-      check(tester.getRect(findBottom)).bottom.equals(900);
+      check(tester.getRect(findBottom)).bottom.equals(600);
+    });
+
+    testWidgets('starts by showing top of bottom sliver, long/long', (tester) async {
+      // Both slivers are long; the bottom sliver gets 75% of the viewport.
+      await prepare(tester, topHeight: 1000, bottomHeight: 3000);
+      check(tester.getRect(findBottom)).top.equals(150);
+    });
+
+    testWidgets('starts by showing top of bottom sliver, short/long', (tester) async {
+      // The top sliver is shorter than 25% of the viewport.
+      // It's shown in full, and the bottom sliver gets the rest (so >75%).
+      await prepare(tester, topHeight: 50, bottomHeight: 3000);
       check(tester.getRect(findTop)).top.equals(0);
+      check(tester.getRect(findBottom)).top.equals(50);
+    });
+
+    testWidgets('starts by showing top of bottom sliver, short/medium', (tester) async {
+      // The whole list fits in the viewport.  It's pinned to the bottom,
+      // even when that gives the bottom sliver more than 75%.
+      await prepare(tester, topHeight: 50, bottomHeight: 500);
+      check(tester.getRect(findTop))..top.equals(50)..bottom.equals(100);
+      check(tester.getRect(findBottom)).bottom.equals(600);
+    });
+
+    testWidgets('starts by showing top of bottom sliver, medium/short', (tester) async {
+      // The whole list fits in the viewport.  It's pinned to the bottom,
+      // even when that gives the top sliver more than 25%.
+      await prepare(tester, topHeight: 300, bottomHeight: 100);
+      check(tester.getRect(findTop))..top.equals(200)..bottom.equals(500);
+      check(tester.getRect(findBottom)).bottom.equals(600);
+    });
+
+    testWidgets('starts by showing top of bottom sliver, long/short', (tester) async {
+      // The bottom sliver is shorter than 75% of the viewport.
+      // It's shown in full, and the top sliver gets the rest (so >25%).
+      await prepare(tester, topHeight: 1000, bottomHeight: 300);
+      check(tester.getRect(findTop)).bottom.equals(300);
+      check(tester.getRect(findBottom)).bottom.equals(600);
     });
 
     testWidgets('short/short -> starts at bottom, immediately without animation', (tester) async {
@@ -204,43 +242,46 @@ void main() {
       check(ys).deepEquals(List.generate(10, (_) => 0.0));
     });
 
-    testWidgets('short/long -> starts at bottom, immediately without animation', (tester) async {
+    testWidgets('short/long -> starts at desired start, immediately without animation', (tester) async {
       await prepare(tester, topHeight: 100, bottomHeight: 800);
 
       final ys = <double>[];
       for (int i = 0; i < 10; i++) {
-        ys.add(tester.getRect(findBottom).bottom - 600);
+        ys.add(tester.getRect(findTop).top);
         await tester.pump(Duration(milliseconds: 15));
       }
       check(ys).deepEquals(List.generate(10, (_) => 0.0));
     });
 
-    testWidgets('starts at bottom, even when bottom underestimated at first', (tester) async {
+    testWidgets('starts at desired start, even when bottom underestimated at first', (tester) async {
       const numItems = 10;
-      const itemHeight = 300.0;
+      const itemHeight = 20.0;
 
       // A list where the bottom sliver takes several rounds of layout
       // to see how long it really is.
       final controller = MessageListScrollController();
       await tester.pumpWidget(Directionality(textDirection: TextDirection.ltr,
         child: MessageListScrollView(
           controller: controller,
+          // The tiny cacheExtent causes each layout round to only reach
+          // the first item it expects will go beyond the viewport.
+          cacheExtent: 1.0, // in (logical) pixels!
           center: const ValueKey('center'),
           slivers: [
             SliverToBoxAdapter(
-              child: SizedBox(height: 100, child: Text('top'))),
+              child: SizedBox(height: 300, child: Text('top'))),
             SliverList.list(key: const ValueKey('center'),
               children: List.generate(numItems, (i) =>
                 SizedBox(height: (i+1) * itemHeight, child: Text('item $i')))),
           ])));
       await tester.pump();
 
-      // Starts out scrolled all the way to the bottom,
-      // even though it must have taken several rounds of layout to find that.
-      check(controller.position.pixels)
-        .equals(itemHeight * numItems * (numItems + 1)/2);
-      check(tester.getRect(find.text('item ${numItems-1}', skipOffstage: false)))
-        .bottom.equals(600);
+      // Starts out with the bottom sliver occupying 75% of the viewport…
+      check(controller.position.pixels).equals(450);
+      // … even though it has more height than that.
+      check(tester.getRect(find.text('item 6'))).bottom.isGreaterThan(600);
+      // (And even though on the first round of layout, it would have looked
+      // much shorter so that the view would have tried to scroll to its end.)
     });
 
     testWidgets('stick to end of list when it grows', (tester) async {