Revert "Changes the offset computation to first item for RenderSliverMainAxisGroup" (flutter#154688) (flutter#168450)

This PR reverts commit 8cc862c

I also added a regression test for
flutter#167801 so that we do not break
that again when revisiting the issue that is reopened.

Fixes flutter#167801
Reopens flutter#154615

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---------

Co-authored-by: Victor Sanni <victorsanniay@gmail.com>

Author: navaronbracke

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

@@ -230,15 +230,7 @@ class RenderSliverMainAxisGroup extends RenderSliver
         double childScrollOffset = 0.0;
         RenderSliver? current = childBefore(child as RenderSliver);
         while (current != null) {
-          // If the current child is not the first child, then we need to
-          // add the scroll extent of the previous child to the current child's
-          // scroll offset.
-          if (childBefore(current) != null) {
-            childScrollOffset +=
-                childAfter(current)!.geometry!.scrollExtent + child.geometry!.scrollExtent;
-          } else if (!(childAfter(child) != null && current.geometry!.hasVisualOverflow)) {
-            childScrollOffset += current.geometry!.scrollExtent;
-          }
+          childScrollOffset += current.geometry!.scrollExtent;
           current = childBefore(current);
         }
         return childScrollOffset;

packages/flutter/test/widgets/sliver_main_axis_group_test.dart

@@ -879,47 +879,78 @@ void main() {
     expect(tester.getTopLeft(find.byKey(key)), Offset.zero);
   });
 
+  // Regression test for https://github.com/flutter/flutter/issues/167801
   testWidgets(
-    'SliverMainAxisGroup scrolls to the correct position when focusing on a text field within a header',
+    'Nesting SliverMainAxisGroups does not break ShowCaretOnScreen for text fields inside nested SliverMainAxisGroup',
     (WidgetTester tester) async {
-      final ScrollController controller = ScrollController();
-      addTearDown(controller.dispose);
-      final FocusNode textFieldFocus = FocusNode();
-      addTearDown(textFieldFocus.dispose);
-      final FocusNode textFieldFocus2 = FocusNode();
-      addTearDown(textFieldFocus2.dispose);
-      const ValueKey<int> firstTextFieldKey = ValueKey<int>(1);
-
-      await tester.pumpWidget(
-        _buildSliverMainAxisGroup(
-          controller: controller,
-          slivers: <Widget>[
-            SliverPersistentHeader(
-              pinned: true,
-              delegate: _SliverTitleDelegate(
-                child: Container(color: Colors.red, height: 60.0),
-                height: 60.0,
+      // The number of groups and items per group needs to be high enough to reproduce the bug.
+      const int sliverGroupsCount = 3;
+      const int sliverGroupItemsCount = 60;
+      // To make working with the scroll offset easier, each item is a fixed height.
+      const double itemHeight = 72.0;
+
+      final ScrollController scrollController = ScrollController();
+      addTearDown(scrollController.dispose);
+
+      final Widget widget = MaterialApp(
+        theme: ThemeData(
+          inputDecorationTheme: const InputDecorationTheme(
+            focusedBorder: OutlineInputBorder(borderSide: BorderSide(color: Color(0xFF1489FD))),
+            enabledBorder: OutlineInputBorder(borderSide: BorderSide(color: Color(0xFFB1BDC5))),
+          ),
+        ),
+        home: Scaffold(
+          body: CustomScrollView(
+            controller: scrollController,
+            slivers: <Widget>[
+              SliverMainAxisGroup(
+                slivers: <Widget>[
+                  for (int i = 1; i <= sliverGroupsCount; i++)
+                    SliverMainAxisGroup(
+                      slivers: <Widget>[
+                        SliverList.builder(
+                          itemCount: sliverGroupItemsCount,
+                          itemBuilder: (_, int index) {
+                            final String label = 'Field $i.${index + 1}';
+
+                            return SizedBox(
+                              height: itemHeight,
+                              child: Padding(
+                                // This extra padding is to make visually debugging the test app a bit better,
+                                // othwerwise the label text clips the text field above.
+                                padding: const EdgeInsets.symmetric(vertical: 8),
+                                child: TextField(
+                                  key: ValueKey<String>(label),
+                                  decoration: InputDecoration(labelText: label),
+                                ),
+                              ),
+                            );
+                          },
+                        ),
+                      ],
+                    ),
+                ],
               ),
-            ),
-            SliverToBoxAdapter(
-              child: Material(child: TextField(key: firstTextFieldKey, focusNode: textFieldFocus)),
-            ),
-            SliverToBoxAdapter(child: Container(color: Colors.green, height: 500)),
-            SliverToBoxAdapter(child: Material(child: TextField(focusNode: textFieldFocus2))),
-          ],
+            ],
+          ),
         ),
       );
 
-      await tester.pumpAndSettle();
-      await tester.pumpAndSettle();
+      await tester.pumpWidget(widget);
+
+      // Scroll down to the first field in the second group, so that it is at the top of the screen.
+      const double offset = sliverGroupItemsCount * itemHeight;
+      scrollController.jumpTo(offset);
 
-      textFieldFocus2.requestFocus();
       await tester.pumpAndSettle();
 
-      textFieldFocus.requestFocus();
+      // Tap the field so that it gains focus and requests the scrollable to scroll it into view.
+      // However, since the field is at the top of the screen, far away from the keyboard,
+      // the scroll position should not change.
+      await tester.tap(find.byKey(const ValueKey<String>('Field 2.1')));
       await tester.pumpAndSettle();
 
-      expect(tester.getTopLeft(find.byKey(firstTextFieldKey)), const Offset(0, 60));
+      expect(scrollController.offset, offset);
     },
   );
 
@@ -1118,22 +1149,3 @@ class TestDelegate extends SliverPersistentHeaderDelegate {
   @override
   bool shouldRebuild(TestDelegate oldDelegate) => true;
 }
-
-class _SliverTitleDelegate extends SliverPersistentHeaderDelegate {
-  _SliverTitleDelegate({required this.height, required this.child});
-  final double height;
-  final Widget child;
-
-  @override
-  double get minExtent => height;
-  @override
-  double get maxExtent => height;
-
-  @override
-  Widget build(BuildContext context, double shrinkOffset, bool overlapsContent) {
-    return child;
-  }
-
-  @override
-  bool shouldRebuild(_SliverTitleDelegate oldDelegate) => true;
-}