From d4da441e53f51ae7a15f0b564fbaba56d60f4673 Mon Sep 17 00:00:00 2001
From: Kate Lovett <katelovett@google.com>
Date: Mon, 21 Dec 2020 18:24:03 -0600
Subject: [PATCH] Apply physics boundary to scrollbar dragging (#72741)

---
 .../flutter/lib/src/widgets/scrollbar.dart    | 12 +++--
 .../flutter/test/widgets/scrollbar_test.dart  | 52 +++++++++++++++++++
 2 files changed, 61 insertions(+), 3 deletions(-)

diff --git a/packages/flutter/lib/src/widgets/scrollbar.dart b/packages/flutter/lib/src/widgets/scrollbar.dart
index 55637cb549..98d9334375 100644
--- a/packages/flutter/lib/src/widgets/scrollbar.dart
+++ b/packages/flutter/lib/src/widgets/scrollbar.dart
@@ -20,6 +20,7 @@ import 'primary_scroll_controller.dart';
 import 'scroll_controller.dart';
 import 'scroll_metrics.dart';
 import 'scroll_notification.dart';
+import 'scroll_position.dart';
 import 'scrollable.dart';
 import 'ticker_provider.dart';
 
@@ -879,13 +880,18 @@ class RawScrollbarState<T extends RawScrollbar> extends State<T> with TickerProv
 
   void _updateScrollPosition(double primaryDelta) {
     assert(_currentController != null);
+    final ScrollPosition position = _currentController!.position;
 
     // Convert primaryDelta, the amount that the scrollbar moved since the last
-    // time _dragScrollbar was called, into the coordinate space of the scroll
+    // time _updateScrollPosition was called, into the coordinate space of the scroll
     // position, and jump to that position.
     final double scrollOffsetLocal = scrollbarPainter.getTrackToScroll(primaryDelta);
-    final double scrollOffsetGlobal = scrollOffsetLocal + _currentController!.position.pixels;
-    _currentController!.position.jumpTo(scrollOffsetGlobal);
+    final double scrollOffsetGlobal = scrollOffsetLocal + position.pixels;
+    if (scrollOffsetGlobal != position.pixels) {
+      // Ensure we don't drag into overscroll if the physics do not allow it.
+      final double physicsAdjustment = position.physics.applyBoundaryConditions(position, scrollOffsetGlobal);
+      position.jumpTo(scrollOffsetGlobal - physicsAdjustment);
+    }
   }
 
   void _maybeStartFadeoutTimer() {
diff --git a/packages/flutter/test/widgets/scrollbar_test.dart b/packages/flutter/test/widgets/scrollbar_test.dart
index 4a6551767d..0b33c60f35 100644
--- a/packages/flutter/test/widgets/scrollbar_test.dart
+++ b/packages/flutter/test/widgets/scrollbar_test.dart
@@ -752,4 +752,56 @@ void main() {
       ),
     );
   });
+
+  testWidgets('Scrollbar thumb cannot be dragged into overscroll if the physics do not allow', (WidgetTester tester) async {
+    final ScrollController scrollController = ScrollController();
+    await tester.pumpWidget(
+      Directionality(
+        textDirection: TextDirection.ltr,
+        child: MediaQuery(
+          data: const MediaQueryData(),
+          child: PrimaryScrollController(
+            controller: scrollController,
+            child: RawScrollbar(
+              isAlwaysShown: true,
+              controller: scrollController,
+              child: const SingleChildScrollView(
+                child: SizedBox(width: 4000.0, height: 4000.0)
+              ),
+            ),
+          ),
+        ),
+      ),
+    );
+    await tester.pumpAndSettle();
+    expect(scrollController.offset, 0.0);
+    expect(
+      find.byType(RawScrollbar),
+      paints
+        ..rect(rect: const Rect.fromLTRB(794.0, 0.0, 800.0, 600.0))
+        ..rect(
+          rect: const Rect.fromLTRB(794.0, 0.0, 800.0, 90.0),
+          color: const Color(0x66BCBCBC),
+        ),
+    );
+
+    // Try to drag the thumb into overscroll.
+    const double scrollAmount = -10.0;
+    final TestGesture dragScrollbarGesture = await tester.startGesture(const Offset(797.0, 45.0));
+    await tester.pumpAndSettle();
+    await dragScrollbarGesture.moveBy(const Offset(0.0, scrollAmount));
+    await tester.pumpAndSettle();
+
+    // The physics should not have allowed us to enter overscroll.
+    expect(scrollController.offset, 0.0);
+    expect(
+      find.byType(RawScrollbar),
+      paints
+        ..rect(rect: const Rect.fromLTRB(794.0, 0.0, 800.0, 600.0))
+        ..rect(
+          rect: const Rect.fromLTRB(794.0, 0.0, 800.0, 90.0),
+          color: const Color(0x66BCBCBC),
+        ),
+    );
+  });
 }