Fix extent for null returning builder in GridView (#132511)

Fixes https://github.com/flutter/flutter/issues/130685

This fixes an issue where a GridView/SliverGrid with a SliverChildBuilderDelegate would still reflect a max scroll extent of infinity after returning null from the builder. This change incorporates the same way we handle this case in SliverList:

https://github.com/flutter/flutter/blob/6d87a23b3722e4fb2e2db993322d5be4adf04fb5/packages/flutter/lib/src/rendering/sliver_list.dart#L305-L307

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

@@ -631,6 +631,7 @@ class RenderSliverGrid extends RenderSliverMultiBoxAdaptor {
     final double leadingScrollOffset = firstChildGridGeometry.scrollOffset;
     double trailingScrollOffset = firstChildGridGeometry.trailingScrollOffset;
     RenderBox? trailingChildWithLayout;
+    bool reachedEnd = false;
 
     for (int index = indexOf(firstChild!) - 1; index >= firstIndex; --index) {
       final SliverGridGeometry gridGeometry = layout.getGeometryForChildIndex(index);
@@ -660,6 +661,7 @@ class RenderSliverGrid extends RenderSliverMultiBoxAdaptor {
       if (child == null || indexOf(child) != index) {
         child = insertAndLayoutChild(childConstraints, after: trailingChildWithLayout);
         if (child == null) {
+          reachedEnd = true;
           // We have run out of children.
           break;
         }
@@ -680,13 +682,15 @@ class RenderSliverGrid extends RenderSliverMultiBoxAdaptor {
     assert(indexOf(firstChild!) == firstIndex);
     assert(targetLastIndex == null || lastIndex <= targetLastIndex);
 
-    final double estimatedTotalExtent = childManager.estimateMaxScrollOffset(
-      constraints,
-      firstIndex: firstIndex,
-      lastIndex: lastIndex,
-      leadingScrollOffset: leadingScrollOffset,
-      trailingScrollOffset: trailingScrollOffset,
-    );
+    final double estimatedTotalExtent = reachedEnd
+      ? trailingScrollOffset
+      : childManager.estimateMaxScrollOffset(
+          constraints,
+          firstIndex: firstIndex,
+          lastIndex: lastIndex,
+          leadingScrollOffset: leadingScrollOffset,
+          trailingScrollOffset: trailingScrollOffset,
+        );
     final double paintExtent = calculatePaintOffset(
       constraints,
       from: math.min(constraints.scrollOffset, leadingScrollOffset),

packages/flutter/test/widgets/grid_view_test.dart

@@ -856,6 +856,43 @@ void main() {
         maxCrossAxisExtent: maxCrossAxisExtent,
       ),
     ), throwsAssertionError);
+  });
+
+  testWidgets('SliverGrid sets correct extent for null returning builder delegate', (WidgetTester tester) async {
+    // Regression test for https://github.com/flutter/flutter/issues/130685
+    final ScrollController controller = ScrollController();
+    await tester.pumpWidget(Directionality(
+      textDirection: TextDirection.ltr,
+      child: GridView.builder(
+        controller: controller,
+        gridDelegate: const SliverGridDelegateWithFixedCrossAxisCount(
+          crossAxisCount: 3,
+          crossAxisSpacing: 16,
+          mainAxisSpacing: 16,
+        ),
+        itemBuilder: (BuildContext context, int index) {
+          if (index == 12) {
+            return null;
+          }
+          return Container(
+            height: 100,
+            width: 100,
+            color: const Color(0xFFFF8A80),
+            alignment: Alignment.center,
+            child: Text('item ${index+1}'),
+          );
+        },
+      ),
+    ));
+    await tester.pumpAndSettle();
 
+    expect(controller.position.maxScrollExtent, double.infinity);
+    expect(controller.position.pixels, 0.0);
+    await tester.fling(find.byType(GridView), const Offset(0.0, -1300.0), 100.0);
+    await tester.pumpAndSettle();
+    // The actual extent of the children is 472.0. This should be reflected when
+    // the builder returns null (meaning we have reached the end).
+    expect(controller.position.maxScrollExtent, 472.0);
+    expect(controller.position.pixels, 472.0);
   });
 }