Refactor SliverMainAxisGroup for reverse mode. (#161849)

Fixes: #159787

We should not refer to `RenderSliverMultiBoxAdaptor`.
`RenderSliverMainAxisGroup` is more like `RenderViewport` because their
children are both Slivers.

*If you had to change anything in the [flutter/tests] repo, include a
link to the migration guide as per the [breaking change policy].*

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [ ] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel
on [Discord].

<!-- Links -->
[Contributor Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#overview
[Tree Hygiene]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md
[test-exempt]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#tests
[Flutter Style Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md
[Features we expect every widget to implement]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md#features-we-expect-every-widget-to-implement
[CLA]: https://cla.developers.google.com/
[flutter/tests]: https://github.com/flutter/tests
[breaking change policy]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#handling-breaking-changes
[Discord]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Chat.md
[Data Driven Fixes]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Data-driven-Fixes.md

---------

Co-authored-by: Kate Lovett <katelovett@google.com>

packages/flutter/lib/src/rendering/sliver_group.dart

@@ -353,57 +353,36 @@ class RenderSliverMainAxisGroup extends RenderSliver
       hasVisualOverflow:
           totalScrollExtent > constraints.remainingPaintExtent || constraints.scrollOffset > 0.0,
     );
+
+    // Update the children's paintOffset based on the direction again, which
+    // must be done after obtaining the `paintExtent`.
+    child = firstChild;
+    while (child != null) {
+      final SliverPhysicalParentData childParentData =
+          child.parentData! as SliverPhysicalParentData;
+      childParentData.paintOffset = switch (constraints.axisDirection) {
+        AxisDirection.up => Offset(
+          0.0,
+          paintExtent - childParentData.paintOffset.dy - child.geometry!.paintExtent,
+        ),
+        AxisDirection.left => Offset(
+          paintExtent - childParentData.paintOffset.dx - child.geometry!.paintExtent,
+          0.0,
+        ),
+        AxisDirection.right || AxisDirection.down => childParentData.paintOffset,
+      };
+      child = childAfter(child);
+    }
   }
 
   @override
   void paint(PaintingContext context, Offset offset) {
-    if (firstChild == null) {
-      return;
-    }
-    // offset is to the top-left corner, regardless of our axis direction.
-    // originOffset gives us the delta from the real origin to the origin in the axis direction.
-    final Offset mainAxisUnit, crossAxisUnit, originOffset;
-    final bool addExtent;
-    switch (applyGrowthDirectionToAxisDirection(
-      constraints.axisDirection,
-      constraints.growthDirection,
-    )) {
-      case AxisDirection.up:
-        mainAxisUnit = const Offset(0.0, -1.0);
-        crossAxisUnit = const Offset(1.0, 0.0);
-        originOffset = offset + Offset(0.0, geometry!.paintExtent);
-        addExtent = true;
-      case AxisDirection.right:
-        mainAxisUnit = const Offset(1.0, 0.0);
-        crossAxisUnit = const Offset(0.0, 1.0);
-        originOffset = offset;
-        addExtent = false;
-      case AxisDirection.down:
-        mainAxisUnit = const Offset(0.0, 1.0);
-        crossAxisUnit = const Offset(1.0, 0.0);
-        originOffset = offset;
-        addExtent = false;
-      case AxisDirection.left:
-        mainAxisUnit = const Offset(-1.0, 0.0);
-        crossAxisUnit = const Offset(0.0, 1.0);
-        originOffset = offset + Offset(geometry!.paintExtent, 0.0);
-        addExtent = true;
-    }
-
     RenderSliver? child = lastChild;
     while (child != null) {
-      final double mainAxisDelta = childMainAxisPosition(child);
-      final double crossAxisDelta = childCrossAxisPosition(child);
-      Offset childOffset = Offset(
-        originOffset.dx + mainAxisUnit.dx * mainAxisDelta + crossAxisUnit.dx * crossAxisDelta,
-        originOffset.dy + mainAxisUnit.dy * mainAxisDelta + crossAxisUnit.dy * crossAxisDelta,
-      );
-      if (addExtent) {
-        childOffset += mainAxisUnit * child.geometry!.paintExtent;
-      }
-
       if (child.geometry!.visible) {
-        context.paintChild(child, childOffset);
+        final SliverPhysicalParentData childParentData =
+            child.parentData! as SliverPhysicalParentData;
+        context.paintChild(child, offset + childParentData.paintOffset);
       }
       child = childBefore(child);
     }

packages/flutter/test/widgets/sliver_main_axis_group_test.dart

@@ -140,12 +140,12 @@ void main() {
     expect(find.text('Group 0 Tile 19'), findsOneWidget);
     expect(
       tester.getRect(find.text('Group 0 Tile 19')),
-      const Rect.fromLTRB(0.0, 0.0, 300.0, 300.0),
+      const Rect.fromLTRB(0.0, 300.0, 300.0, 600.0),
     );
     expect(find.text('Group 1 Tile 0'), findsOneWidget);
     expect(
       tester.getRect(find.text('Group 1 Tile 0')),
-      const Rect.fromLTRB(0.0, 400.0, 300.0, 600.0),
+      const Rect.fromLTRB(0.0, 100.0, 300.0, 300.0),
     );
 
     final List<RenderSliverList> renderSlivers =
@@ -158,9 +158,9 @@ void main() {
     expect(first.geometry!.scrollExtent, equals(20 * 300.0));
     expect(second.geometry!.scrollExtent, equals(20 * 200.0));
 
-    expect((first.parentData! as SliverPhysicalParentData).paintOffset.dy, equals(0.0));
+    expect((first.parentData! as SliverPhysicalParentData).paintOffset.dy, equals(300.0));
     expect(first.constraints.scrollOffset, equals(19 * 300.0));
-    expect((second.parentData! as SliverPhysicalParentData).paintOffset.dy, equals(1 * 300.0));
+    expect((second.parentData! as SliverPhysicalParentData).paintOffset.dy, equals(0.0));
 
     final RenderSliverMainAxisGroup renderGroup = tester.renderObject<RenderSliverMainAxisGroup>(
       find.byType(SliverMainAxisGroup),
@@ -936,6 +936,25 @@ void main() {
     expect(find.text('1'), findsNothing);
     expect(find.text('1', skipOffstage: false), findsOneWidget);
   });
+
+  testWidgets("The localToGlobal of SliverMainAxisGroup's children works in reverse.", (
+    WidgetTester tester,
+  ) async {
+    await tester.pumpWidget(
+      _buildSliverMainAxisGroup(
+        viewportHeight: 400,
+        reverse: true,
+        slivers: <Widget>[
+          const SliverToBoxAdapter(child: SizedBox(height: 70)),
+          const SliverToBoxAdapter(child: SizedBox(height: 20, child: Text('1'))),
+          const SliverToBoxAdapter(child: SizedBox(height: 700)),
+        ],
+      ),
+    );
+    final RenderBox renderBox = tester.renderObject(find.text('1')) as RenderBox;
+    expect(renderBox.localToGlobal(Offset.zero), const Offset(0.0, 310.0));
+    expect(tester.getTopLeft(find.text('1')), const Offset(0.0, 310.0));
+  });
 }
 
 Widget _buildSliverList({