scroll: Start out scrolled to bottom of list

This is NFC as to the real message list, because so far the bottom
sliver there always has height 0, so that maxScrollExtent is always 0.

This is a step toward letting us move part of the message list into
the bottom sliver, because it means that doing so would preserve the
list's current behavior of starting out scrolled to the end.

Author: gnprice

lib/widgets/scrolling.dart

@@ -260,6 +260,14 @@ class MessageListScrollPosition extends ScrollPositionWithSingleContext {
     super.debugLabel,
   });
 
+  // TODO(upstream): is the lack of [absorb] a bug in [_TabBarScrollPosition]?
+  @override
+  void absorb(ScrollPosition other) {
+    super.absorb(other);
+    if (other is! MessageListScrollPosition) return;
+    _hasEverCompletedLayout = other._hasEverCompletedLayout;
+  }
+
   /// Like [applyContentDimensions], but called without adjusting
   /// the arguments to subtract the viewport dimension.
   ///
@@ -278,6 +286,36 @@ class MessageListScrollPosition extends ScrollPositionWithSingleContext {
     final effectiveMax = wholeMaxScrollExtent;
     return applyContentDimensions(effectiveMin, effectiveMax);
   }
+
+  bool _hasEverCompletedLayout = false;
+
+  @override
+  bool applyContentDimensions(double minScrollExtent, double maxScrollExtent) {
+    // Inspired by _TabBarScrollPosition.applyContentDimensions upstream.
+    bool changed = false;
+
+    if (!_hasEverCompletedLayout) {
+      // The list is being laid out for the first time (its first performLayout).
+      // Start out scrolled to the end.
+      final target = maxScrollExtent;
+      if (!hasPixels || pixels != target) {
+        correctPixels(target);
+        changed = true;
+      }
+    }
+
+    if (!super.applyContentDimensions(minScrollExtent, maxScrollExtent)) {
+      changed = true;
+    }
+
+    if (!changed) {
+      // Because this method is about to return true,
+      // this will be the last round of this layout.
+      _hasEverCompletedLayout = true;
+    }
+
+    return !changed;
+  }
 }
 
 /// A version of [ScrollController] adapted for the Zulip message list.

test/widgets/scrolling_test.dart

@@ -6,6 +6,8 @@ import 'package:flutter/widgets.dart' hide SliverPaintOrder;
 import 'package:flutter_test/flutter_test.dart';
 import 'package:zulip/widgets/scrolling.dart';
 
+import '../flutter_checks.dart';
+
 void main() {
   group('CustomPaintOrderScrollView paint order', () {
     final paintLog = <int>[];
@@ -124,6 +126,126 @@ void main() {
                    //   the former matched the latter's old default behavior.
     );
   });
+
+  group('MessageListScrollView', () {
+    Widget buildList({
+      MessageListScrollController? controller,
+      required double topHeight,
+      required double bottomHeight,
+    }) {
+      return MessageListScrollView(
+        controller: controller ?? MessageListScrollController(),
+        center: const ValueKey('center'),
+        slivers: [
+          SliverToBoxAdapter(
+            child: SizedBox(height: topHeight, child: Text('top'))),
+          SliverToBoxAdapter(key: const ValueKey('center'),
+            child: SizedBox(height: bottomHeight, child: Text('bottom'))),
+        ]);
+    }
+
+    Future<void> prepare(WidgetTester tester, {
+      MessageListScrollController? controller,
+      required double topHeight,
+      required double bottomHeight,
+    }) async {
+      await tester.pumpWidget(Directionality(textDirection: TextDirection.ltr,
+        child: buildList(controller: controller,
+          topHeight: topHeight, bottomHeight: bottomHeight)));
+      await tester.pump();
+    }
+
+    // The `skipOffstage: false` produces more informative output
+    // when a test fails because one of the slivers is just offscreen.
+    final findTop = find.text('top', skipOffstage: false);
+    final findBottom = find.text('bottom', skipOffstage: false);
+
+    testWidgets('short/short -> starts scrolled to bottom', (tester) async {
+      // Starts out with items at bottom of viewport.
+      await prepare(tester, topHeight: 100, bottomHeight: 100);
+      check(tester.getRect(findBottom)).bottom.equals(600);
+
+      // Try scrolling down (by dragging up); doesn't move.
+      await tester.drag(findTop, Offset(0, -100));
+      await tester.pump();
+      check(tester.getRect(findBottom)).bottom.equals(600);
+    });
+
+    testWidgets('short/long -> starts scrolled to bottom', (tester) async {
+      // Starts out scrolled to bottom.
+      await prepare(tester, topHeight: 100, bottomHeight: 800);
+      check(tester.getRect(findBottom)).bottom.equals(600);
+
+      // Try scrolling down (by dragging up); doesn't move.
+      await tester.drag(findBottom, Offset(0, -100));
+      await tester.pump();
+      check(tester.getRect(findBottom)).bottom.equals(600);
+    });
+
+    testWidgets('starts at bottom, even when bottom underestimated at first', (tester) async {
+      const numItems = 10;
+      const itemHeight = 300.0;
+
+      // 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,
+          center: const ValueKey('center'),
+          slivers: [
+            SliverToBoxAdapter(
+              child: SizedBox(height: 100, 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);
+    });
+
+    testWidgets('position preserved when scrollable rebuilds', (tester) async {
+      // Tests that [MessageListScrollPosition.absorb] does its job.
+      //
+      // In the app, this situation can be triggered by changing the device's
+      // theme between light and dark.  For this simplified example for a test,
+      // go for devicePixelRatio (which ScrollableState directly depends on).
+
+      final controller = MessageListScrollController();
+      final widget = Directionality(textDirection: TextDirection.ltr,
+        child: buildList(controller: controller,
+          topHeight: 400, bottomHeight: 400));
+      await tester.pumpWidget(
+        MediaQuery(data: MediaQueryData(devicePixelRatio: 1.0),
+          child: widget));
+      check(tester.getRect(findTop)).bottom.equals(200);
+      final position = controller.position;
+      check(position).isA<MessageListScrollPosition>();
+
+      // Drag away from the initial scroll position.
+      await tester.drag(findBottom, Offset(0, 200));
+      await tester.pump();
+      check(tester.getRect(findTop)).bottom.equals(400);
+      check(controller.position).identicalTo(position);
+
+      // Then cause the ScrollableState to have didChangeDependencies called…
+      await tester.pumpWidget(
+        MediaQuery(data: MediaQueryData(devicePixelRatio: 2.0),
+          child: widget));
+      // … so that it constructs a new MessageListScrollPosition…
+      check(controller.position)
+        ..not((it) => it.identicalTo(position))
+        ..isA<MessageListScrollPosition>();
+      // … and check the scroll position is preserved, not reset to initial.
+      check(tester.getRect(findTop)).bottom.equals(400);
+    });
+  });
 }
 
 class TestCustomPainter extends CustomPainter {