Fix SliverMainAxisGroup layout in reverse (#145572)

Fixes https://github.com/flutter/flutter/issues/145068

The original tests for SliverMainAxisGroup did not actually check where the child was painted (#126596).

Followed the same pattern in RenderSliverMultiBoxAdaptor:

11c034f037/packages/flutter/lib/src/rendering/sliver_multi_box_adaptor.dart (L666)

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

@@ -274,8 +274,8 @@ class RenderSliverMainAxisGroup extends RenderSliver with ContainerRenderObjectM
       assert(() {
         if (child != null && maxPaintExtent.isInfinite) {
           throw FlutterError(
-            'Unreachable sliver found, you may have a sliver behind '
+            'Unreachable sliver found, you may have a sliver following '
-            'a sliver with infinite extent. '
+            'a sliver with an infinite extent. '
           );
         }
         return true;
@@ -327,12 +327,50 @@ class RenderSliverMainAxisGroup extends RenderSliver with ContainerRenderObjectM
 
   @override
   void paint(PaintingContext context, Offset offset) {
-    RenderSliver? child = lastChild;
+    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) {
-        final SliverPhysicalParentData childParentData = child.parentData! as SliverPhysicalParentData;
+        context.paintChild(child, childOffset);
-        context.paintChild(child, offset + childParentData.paintOffset);
       }
       child = childBefore(child);
     }

packages/flutter/test/widgets/sliver_main_axis_group_test.dart

@@ -32,9 +32,16 @@ void main() {
     expect(controller.offset, 0);
 
     expect(find.text('Group 0 Tile 0'), findsOneWidget);
+    expect(
+      tester.getRect(find.text('Group 0 Tile 0')),
+      const Rect.fromLTRB(0.0, 0.0, 300.0, 300.0),
+    );
     expect(find.text('Group 0 Tile 1'), findsOneWidget);
+    expect(
+      tester.getRect(find.text('Group 0 Tile 1')),
+      const Rect.fromLTRB(0.0, 300.0, 300.0, 600.0),
+    );
     expect(find.text('Group 0 Tile 2'), findsNothing);
-
     expect(find.text('Group 1 Tile 0'), findsNothing);
 
     const double scrollOffset = 19 * 300.0;
@@ -44,7 +51,15 @@ void main() {
     expect(controller.offset, scrollOffset);
     expect(find.text('Group 0 Tile 18'), findsNothing);
     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),
+    );
     expect(find.text('Group 1 Tile 0'), findsOneWidget);
+    expect(
+      tester.getRect(find.text('Group 1 Tile 0')),
+      const Rect.fromLTRB(0.0, 300.0, 300.0, 500.0),
+    );
 
     final List<RenderSliverList> renderSlivers = tester.renderObjectList<RenderSliverList>(find.byType(SliverList)).toList();
     final RenderSliverList first = renderSlivers[0];
@@ -85,9 +100,16 @@ void main() {
     expect(controller.offset, 0);
 
     expect(find.text('Group 0 Tile 0'), findsOneWidget);
+    expect(
+      tester.getRect(find.text('Group 0 Tile 0')),
+      const Rect.fromLTRB(0.0, 300.0, 300.0, 600.0),
+    );
     expect(find.text('Group 0 Tile 1'), findsOneWidget);
+    expect(
+      tester.getRect(find.text('Group 0 Tile 1')),
+      const Rect.fromLTRB(0.0, 0.0, 300.0, 300.0),
+    );
     expect(find.text('Group 0 Tile 2'), findsNothing);
-
     expect(find.text('Group 1 Tile 0'), findsNothing);
 
     const double scrollOffset = 19 * 300.0;
@@ -97,7 +119,15 @@ void main() {
     expect(controller.offset, scrollOffset);
     expect(find.text('Group 0 Tile 18'), findsNothing);
     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),
+    );
     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),
+    );
 
     final List<RenderSliverList> renderSlivers = tester.renderObjectList<RenderSliverList>(find.byType(SliverList)).toList();
     final RenderSliverList first = renderSlivers[0];
@@ -138,8 +168,11 @@ void main() {
     expect(controller.offset, 0);
 
     expect(find.text('Group 0 Tile 0'), findsOneWidget);
+    expect(
+      tester.getRect(find.text('Group 0 Tile 0')),
+      const Rect.fromLTRB(0.0, 0.0, 300.0, 600.0),
+    );
     expect(find.text('Group 0 Tile 1'), findsNothing);
-
     expect(find.text('Group 1 Tile 0'), findsNothing);
 
     const double scrollOffset = 19 * 300.0;
@@ -149,6 +182,10 @@ void main() {
     expect(controller.offset, scrollOffset);
     expect(find.text('Group 0 Tile 18'), findsNothing);
     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, 600.0),
+    );
     expect(find.text('Group 1 Tile 0'), findsNothing);
 
     const double scrollOffset2 = 20 * 300.0;
@@ -156,6 +193,10 @@ void main() {
     await tester.pumpAndSettle();
     expect(find.text('Group 0 Tile 19'), findsNothing);
     expect(find.text('Group 1 Tile 0'), findsOneWidget);
+    expect(
+      tester.getRect(find.text('Group 1 Tile 0')),
+      const Rect.fromLTRB(0.0, 0.0, 200.0, 600.0),
+    );
 
     final List<RenderSliverList> renderSlivers = tester.renderObjectList<RenderSliverList>(find.byType(SliverList)).toList();
     final RenderSliverList first = renderSlivers[0];
@@ -197,8 +238,11 @@ void main() {
     expect(controller.offset, 0);
 
     expect(find.text('Group 0 Tile 0'), findsOneWidget);
+    expect(
+      tester.getRect(find.text('Group 0 Tile 0')),
+      const Rect.fromLTRB(0.0, 0.0, 300.0, 600.0),
+    );
     expect(find.text('Group 0 Tile 1'), findsNothing);
-
     expect(find.text('Group 1 Tile 0'), findsNothing);
 
     const double scrollOffset = 19 * 300.0;
@@ -208,6 +252,10 @@ void main() {
     expect(controller.offset, scrollOffset);
     expect(find.text('Group 0 Tile 18'), findsNothing);
     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, 600.0),
+    );
     expect(find.text('Group 1 Tile 0'), findsNothing);
 
     const double scrollOffset2 = 20 * 300.0;
@@ -215,6 +263,10 @@ void main() {
     await tester.pumpAndSettle();
     expect(find.text('Group 0 Tile 19'), findsNothing);
     expect(find.text('Group 1 Tile 0'), findsOneWidget);
+    expect(
+      tester.getRect(find.text('Group 1 Tile 0')),
+      const Rect.fromLTRB(100.0, 0.0, 300.0, 600.0),
+    );
 
     final List<RenderSliverList> renderSlivers = tester.renderObjectList<RenderSliverList>(find.byType(SliverList)).toList();
     final RenderSliverList first = renderSlivers[0];