Fix the issue with SliverMainAxisGroups growing in the reverse direction during layout. (#171005)

Fixes: #170991

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- [ ] 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].
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packages/flutter/lib/src/rendering/sliver_group.dart

@@ -263,7 +263,14 @@ class RenderSliverMainAxisGroup extends RenderSliver
     double maxPaintExtent = 0;
     double paintOffset = constraints.overlap;
 
-    RenderSliver? child = firstChild;
+    final (
+      RenderSliver? leadingChild,
+      RenderSliver? Function(RenderSliver child) advance,
+    ) = switch (constraints.growthDirection) {
+      GrowthDirection.forward => (firstChild, childAfter),
+      GrowthDirection.reverse => (lastChild, childBefore),
+    };
+    RenderSliver? child = leadingChild;
     while (child != null) {
       final double beforeOffsetPaintExtent = calculatePaintOffset(
         constraints,
@@ -298,7 +305,7 @@ class RenderSliverMainAxisGroup extends RenderSliver
       layoutOffset += childLayoutGeometry.layoutExtent;
       maxPaintExtent += childLayoutGeometry.maxPaintExtent;
       paintOffset = math.max(childPaintOffset + childLayoutGeometry.paintExtent, paintOffset);
-      child = childAfter(child);
+      child = advance(child);
       assert(() {
         if (child != null && maxPaintExtent.isInfinite) {
           throw FlutterError(
@@ -349,11 +356,14 @@ class RenderSliverMainAxisGroup extends RenderSliver
 
     // Update the children's paintOffset based on the direction again, which
     // must be done after obtaining the `paintExtent`.
-    child = firstChild;
+    child = leadingChild;
     while (child != null) {
       final SliverPhysicalParentData childParentData =
           child.parentData! as SliverPhysicalParentData;
-      childParentData.paintOffset = switch (constraints.axisDirection) {
+      childParentData.paintOffset = switch (applyGrowthDirectionToAxisDirection(
+        constraints.axisDirection,
+        constraints.growthDirection,
+      )) {
         AxisDirection.up => Offset(
           0.0,
           paintExtent - childParentData.paintOffset.dy - child.geometry!.paintExtent,
@@ -364,7 +374,7 @@ class RenderSliverMainAxisGroup extends RenderSliver
         ),
         AxisDirection.right || AxisDirection.down => childParentData.paintOffset,
       };
-      child = childAfter(child);
+      child = advance(child);
     }
   }
 

packages/flutter/test/widgets/sliver_main_axis_group_test.dart

@@ -1062,6 +1062,41 @@ void main() {
     await tester.pumpAndSettle();
     expect(tester.takeException(), isNull);
   });
+
+  testWidgets('SliverMainAxisGroup with center', (WidgetTester tester) async {
+    final ScrollController controller = ScrollController();
+    addTearDown(controller.dispose);
+    const Key centerKey = Key('center');
+    await tester.pumpWidget(
+      MaterialApp(
+        home: CustomScrollView(
+          center: centerKey,
+          controller: controller,
+          slivers: const <Widget>[
+            SliverMainAxisGroup(
+              slivers: <Widget>[
+                SliverToBoxAdapter(child: SizedBox(height: 50, child: Text('-2'))),
+                SliverToBoxAdapter(child: SizedBox(height: 50, child: Text('-1'))),
+              ],
+            ),
+            SliverMainAxisGroup(
+              key: centerKey,
+              slivers: <Widget>[
+                SliverToBoxAdapter(child: SizedBox(height: 50, child: Text('1'))),
+                SliverToBoxAdapter(child: SizedBox(height: 50, child: Text('2'))),
+              ],
+            ),
+          ],
+        ),
+      ),
+    );
+    controller.jumpTo(-51);
+    await tester.pumpAndSettle();
+    expect(tester.getTopLeft(find.text('-1')), const Offset(0, 1));
+    expect(tester.getTopLeft(find.text('1')), const Offset(0, 51));
+    expect(tester.getTopLeft(find.text('2')), const Offset(0, 101));
+    expect(tester.getTopLeft(find.text('-2')), const Offset(0, -49));
+  });
 }
 
 Widget _buildSliverList({