fix sliverfixedextent with sliverchildbuilderdelegate does not correct calculate max scroll extent (#39142)

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

@@ -195,7 +195,24 @@ abstract class RenderSliverFixedExtentBoxAdaptor extends RenderSliverMultiBoxAda
     if (firstChild == null) {
       if (!addInitialChild(index: firstIndex, layoutOffset: indexToLayoutOffset(itemExtent, firstIndex))) {
         // There are either no children, or we are past the end of all our children.
-        final double max = computeMaxScrollOffset(constraints, itemExtent);
+        // If it is the later, we will need to find the first available child.
+        double max;
+        if (childManager.childCount != null) {
+          max = computeMaxScrollOffset(constraints, itemExtent);
+        } else if (firstIndex <= 0) {
+          max = 0.0;
+        } else {
+          // We will have to find it manually.
+          int possibleFirstIndex = firstIndex - 1;
+          while (possibleFirstIndex > 0 &&
+            !addInitialChild(
+              index: possibleFirstIndex,
+              layoutOffset: indexToLayoutOffset(itemExtent, possibleFirstIndex)
+            )
+          )
+            possibleFirstIndex -= 1;
+          max = possibleFirstIndex * itemExtent;
+        }
         geometry = SliverGeometry(
           scrollExtent: max,
           maxPaintExtent: max,
@@ -229,12 +246,14 @@ abstract class RenderSliverFixedExtentBoxAdaptor extends RenderSliverMultiBoxAda
       trailingChildWithLayout = firstChild;
     }
 
+    double estimatedMaxScrollOffset = double.infinity;
     for (int index = indexOf(trailingChildWithLayout) + 1; targetLastIndex == null || index <= targetLastIndex; ++index) {
       RenderBox child = childAfter(trailingChildWithLayout);
       if (child == null || indexOf(child) != index) {
         child = insertAndLayoutChild(childConstraints, after: trailingChildWithLayout);
         if (child == null) {
           // We have run out of children.
+          estimatedMaxScrollOffset = index * itemExtent;
           break;
         }
       } else {
@@ -256,12 +275,15 @@ abstract class RenderSliverFixedExtentBoxAdaptor extends RenderSliverMultiBoxAda
     assert(indexOf(firstChild) == firstIndex);
     assert(targetLastIndex == null || lastIndex <= targetLastIndex);
 
-    final double estimatedMaxScrollOffset = estimateMaxScrollOffset(
+    estimatedMaxScrollOffset = math.min(
-      constraints,
+      estimatedMaxScrollOffset,
-      firstIndex: firstIndex,
+      estimateMaxScrollOffset(
-      lastIndex: lastIndex,
+        constraints,
-      leadingScrollOffset: leadingScrollOffset,
+        firstIndex: firstIndex,
-      trailingScrollOffset: trailingScrollOffset,
+        lastIndex: lastIndex,
+        leadingScrollOffset: leadingScrollOffset,
+        trailingScrollOffset: trailingScrollOffset,
+      )
     );
 
     final double paintExtent = calculatePaintOffset(

packages/flutter/test/widgets/slivers_test.dart

@@ -350,6 +350,70 @@ void main() {
     expect(tester.takeException(), 'builder');
     ErrorWidget.builder = oldBuilder;
   });
+
+  testWidgets('SliverFixedExtentList with SliverChildBuilderDelegate auto-correct scroll offset - super fast', (WidgetTester tester) async {
+    final ScrollController controller = ScrollController(initialScrollOffset: 600);
+    await tester.pumpWidget(
+      Directionality(
+        textDirection: TextDirection.ltr,
+        child: CustomScrollView(
+          controller: controller,
+          cacheExtent: 0,
+          slivers: <Widget>[
+            SliverFixedExtentList(
+              itemExtent: 200,
+              delegate: SliverChildBuilderDelegate(
+                  (BuildContext context, int index) {
+                  if (index <= 6) {
+                    return Center(child: Text('Page $index'));
+                  }
+                  return null;
+                },
+              ),
+            )
+          ],
+        ),
+      )
+    );
+    await tester.drag(find.text('Page 5'), const Offset(0, -1000));
+    // Controller will be temporarily over-scrolled.
+    expect(controller.offset, 1600.0);
+    await tester.pumpAndSettle();
+    // It will be corrected after a auto scroll animation.
+    expect(controller.offset, 800.0);
+  });
+
+  testWidgets('SliverFixedExtentList with SliverChildBuilderDelegate auto-correct scroll offset - reasonable', (WidgetTester tester) async {
+    final ScrollController controller = ScrollController(initialScrollOffset: 600);
+    await tester.pumpWidget(
+      Directionality(
+        textDirection: TextDirection.ltr,
+        child: CustomScrollView(
+          controller: controller,
+          cacheExtent: 0,
+          slivers: <Widget>[
+            SliverFixedExtentList(
+              itemExtent: 200,
+              delegate: SliverChildBuilderDelegate(
+                  (BuildContext context, int index) {
+                  if (index <= 6) {
+                    return Center(child: Text('Page $index'));
+                  }
+                  return null;
+                },
+              ),
+            )
+          ],
+        ),
+      )
+    );
+    await tester.drag(find.text('Page 5'), const Offset(0, -210));
+    // Controller will be temporarily over-scrolled.
+    expect(controller.offset, 810.0);
+    await tester.pumpAndSettle();
+    // It will be corrected after a auto scroll animation.
+    expect(controller.offset, 800.0);
+  });
 }
 
 bool isRight(Offset a, Offset b) => b.dx > a.dx;