RangeMaintainingScrollPhysics remove overscroll maintaining when grow (#90608)

packages/flutter/lib/src/widgets/scroll_physics.dart

@@ -428,8 +428,12 @@ class ScrollPhysics {
 /// Scroll physics that attempt to keep the scroll position in range when the
 /// contents change dimensions suddenly.
 ///
-/// If the scroll position is already out of range, this attempts to maintain
-/// the amount of overscroll or underscroll already present.
+/// This attempts to maintain the amount of overscroll or underscroll already present,
+/// if the scroll position is already out of range _and_ the extents
+/// have decreased, meaning that some content was removed. The reason for this
+/// condition is that when new content is added, keeping the same overscroll
+/// would mean that instead of showing it to the user, all of it is
+/// being skipped by jumping right to the max extent.
 ///
 /// If the scroll activity is animating the scroll position, sudden changes to
 /// the scroll dimensions are allowed to happen (so as to prevent animations
@@ -513,9 +517,9 @@ class RangeMaintainingScrollPhysics extends ScrollPhysics {
       maintainOverscroll = false;
       if (oldPosition.minScrollExtent.isFinite && oldPosition.maxScrollExtent.isFinite &&
           newPosition.minScrollExtent.isFinite && newPosition.maxScrollExtent.isFinite) {
-        // In addition, if the position changed then we only enforce
-        // the new boundary if the previous boundary was not entirely
-        // finite. A common case where the position changes while one
+        // In addition, if the position changed then we don't enforce the new
+        // boundary if both the new and previous boundaries are entirely finite.
+        // A common case where the position changes while one
         // of the extents is infinite is a lazily-loaded list. (If the
         // boundaries were finite, and the position changed, then we
         // assume it was intentional.)
@@ -529,13 +533,19 @@ class RangeMaintainingScrollPhysics extends ScrollPhysics {
       enforceBoundary = false;
     }
     if (maintainOverscroll) {
-      // Force the new position to be no more out of range
-      // than it was before, if it was overscrolled.
-      if (oldPosition.pixels < oldPosition.minScrollExtent) {
+      // Force the new position to be no more out of range than it was before, if:
+      //  * it was overscrolled, and
+      //  * the extents have decreased, meaning that some content was removed. The
+      //    reason for this condition is that when new content is added, keeping
+      //    the same overscroll would mean that instead of showing it to the user,
+      //    all of it is being skipped by jumping right to the max extent.
+      if (oldPosition.pixels < oldPosition.minScrollExtent &&
+          newPosition.minScrollExtent > oldPosition.minScrollExtent) {
         final double oldDelta = oldPosition.minScrollExtent - oldPosition.pixels;
         return newPosition.minScrollExtent - oldDelta;
       }
-      if (oldPosition.pixels > oldPosition.maxScrollExtent) {
+      if (oldPosition.pixels > oldPosition.maxScrollExtent &&
+          newPosition.maxScrollExtent < oldPosition.maxScrollExtent) {
         final double oldDelta = oldPosition.pixels - oldPosition.maxScrollExtent;
         return newPosition.maxScrollExtent + oldDelta;
       }

packages/flutter/test/widgets/range_maintaining_scroll_physics_test.dart

@@ -230,7 +230,7 @@ void main() {
     expect(bike2.center, bike1.shift(const Offset(100.0, 0.0)).center);
   });
 
-  testWidgets('Changing the size of the viewport while you are overdragged', (WidgetTester tester) async {
+  testWidgets('changing the size of the viewport when overscrolled', (WidgetTester tester) async {
     Widget build(double height) {
       return Directionality(
         textDirection: TextDirection.rtl,
@@ -263,6 +263,82 @@ void main() {
     final Rect newPosition = tester.getRect(find.byType(Placeholder));
     expect(oldPosition, newPosition);
   });
+
+  testWidgets('inserting and removing an item when overscrolled', (WidgetTester tester) async {
+    // Regression test for https://github.com/flutter/flutter/issues/62890
+
+    const double itemExtent = 100.0;
+    final UniqueKey key = UniqueKey();
+    final Finder finder = find.byKey(key);
+    Widget build({required bool twoItems}) {
+      return Directionality(
+        textDirection: TextDirection.rtl,
+        child: ScrollConfiguration(
+          behavior: const RangeMaintainingTestScrollBehavior(),
+          child: Align(
+            child: SizedBox(
+              width: 100.0,
+              height: 100.0,
+              child: ListView(
+                children: <Widget>[
+                  SizedBox(height: itemExtent, child: Placeholder(key: key)),
+                  if (twoItems)
+                    const SizedBox(height: itemExtent, child: Placeholder()),
+                ],
+              ),
+            ),
+          ),
+        ),
+      );
+    }
+
+    await tester.pumpWidget(build(twoItems: false));
+    final ScrollPosition position = tester.state<ScrollableState>(find.byType(Scrollable)).position;
+
+    // overscroll bottom
+    final TestGesture drag1 = await tester.startGesture(tester.getCenter(finder));
+    await tester.pump();
+    await drag1.moveBy(const Offset(0.0, -50.0));
+    await tester.pump();
+
+    final double oldOverscroll1 = position.pixels - position.maxScrollExtent;
+    final Rect oldPosition1 = tester.getRect(finder);
+    await tester.pumpWidget(build(twoItems: true));
+    // verify inserting new item didn't change the position of the first one
+    expect(oldPosition1, tester.getRect(finder));
+    // verify the overscroll changed by the size of the added item
+    final double newOverscroll1 = position.pixels - position.maxScrollExtent;
+    expect(oldOverscroll1, isPositive);
+    expect(newOverscroll1, isNegative);
+    expect(newOverscroll1, oldOverscroll1 - itemExtent);
+
+    await drag1.up();
+
+    // verify there's no ballistic animation, because we weren't overscrolled
+    expect(await tester.pumpAndSettle(), 1);
+
+    // overscroll bottom
+    final TestGesture drag2 = await tester.startGesture(tester.getCenter(finder));
+    await tester.pump();
+    await drag2.moveBy(const Offset(0.0, -100.0));
+    await tester.pump();
+
+    final double oldOverscroll2 = position.pixels - position.maxScrollExtent;
+    // should find nothing because item is not visible
+    expect(finder, findsNothing);
+    await tester.pumpWidget(build(twoItems: false));
+    // verify removing an item changed the position of the first one, because prior it was not visible
+    expect(oldPosition1, tester.getRect(finder));
+    // verify the overscroll was maintained
+    final double newOverscroll2 = position.pixels - position.maxScrollExtent;
+    expect(oldOverscroll2, isPositive);
+    expect(oldOverscroll2, newOverscroll2);
+
+    await drag2.up();
+
+    // verify there's a ballistic animation from overscroll
+    expect(await tester.pumpAndSettle(), 9);
+  });
 }
 
 class TabBarDemo extends StatelessWidget {

packages/flutter/test/widgets/scroll_activity_test.dart

@@ -13,7 +13,7 @@ List<Widget> children(int n) {
 }
 
 void main() {
-  testWidgets('Scrolling with list view changes', (WidgetTester tester) async {
+  testWidgets('Scrolling with list view changes, leaving the overscroll', (WidgetTester tester) async {
     final ScrollController controller = ScrollController();
     await tester.pumpWidget(MaterialApp(home: ListView(controller: controller, children: children(30))));
     final double thirty = controller.position.maxScrollExtent;
@@ -22,7 +22,21 @@ void main() {
     controller.jumpTo(thirty + 100.0); // past the end
     await tester.pump();
     await tester.pumpWidget(MaterialApp(home: ListView(controller: controller, children: children(31))));
-    expect(controller.position.pixels, thirty + 200.0); // same distance past the end
+    expect(controller.position.pixels, thirty + 100.0); // has the same position, but no longer overscrolled
+    expect(await tester.pumpAndSettle(), 1); // doesn't have ballistic animation...
+    expect(controller.position.pixels, thirty + 100.0); // and ends up at the end
+  });
+
+  testWidgets('Scrolling with list view changes, remaining overscrolled', (WidgetTester tester) async {
+    final ScrollController controller = ScrollController();
+    await tester.pumpWidget(MaterialApp(home: ListView(controller: controller, children: children(30))));
+    final double thirty = controller.position.maxScrollExtent;
+    controller.jumpTo(thirty);
+    await tester.pump();
+    controller.jumpTo(thirty + 200.0); // past the end
+    await tester.pump();
+    await tester.pumpWidget(MaterialApp(home: ListView(controller: controller, children: children(31))));
+    expect(controller.position.pixels, thirty + 200.0); // has the same position, still overscrolled
     expect(await tester.pumpAndSettle(), 8); // now it goes ballistic...
     expect(controller.position.pixels, thirty + 100.0); // and ends up at the end
   });