From 12b8d9db80e9ed41c7a9818fa5df137dd07b4746 Mon Sep 17 00:00:00 2001
From: Pascal Welsch <pascal@welsch.dev>
Date: Sat, 8 Aug 2020 05:21:07 +0200
Subject: [PATCH] Reduce iOS scroll damping for lists with differently sized
 items (#59623)

---
 .../lib/src/widgets/scroll_physics.dart       |  7 +++----
 .../lib/src/widgets/scroll_simulation.dart    |  2 ++
 .../test/widgets/scroll_physics_test.dart     | 20 +++++++++++++++++++
 3 files changed, 25 insertions(+), 4 deletions(-)

diff --git a/packages/flutter/lib/src/widgets/scroll_physics.dart b/packages/flutter/lib/src/widgets/scroll_physics.dart
index 1c1c72c590..1e9b990123 100644
--- a/packages/flutter/lib/src/widgets/scroll_physics.dart
+++ b/packages/flutter/lib/src/widgets/scroll_physics.dart
@@ -533,7 +533,7 @@ class BouncingScrollPhysics extends ScrollPhysics {
       return BouncingScrollSimulation(
         spring: spring,
         position: position.pixels,
-        velocity: velocity * 0.91, // TODO(abarth): We should move this constant closer to the drag end.
+        velocity: velocity,
         leadingExtent: position.minScrollExtent,
         trailingExtent: position.maxScrollExtent,
         tolerance: tolerance,
@@ -549,9 +549,8 @@ class BouncingScrollPhysics extends ScrollPhysics {
   double get minFlingVelocity => kMinFlingVelocity * 2.0;
 
   // Methodology:
-  // 1- Use https://github.com/flutter/scroll_overlay to test with Flutter and
-  //    platform scroll views superimposed.
-  // 2- Record incoming speed and make rapid flings in the test app.
+  // 1- Use https://github.com/flutter/platform_tests/tree/master/scroll_overlay to test with
+  //    Flutter and platform scroll views superimposed.
   // 3- If the scrollables stopped overlapping at any moment, adjust the desired
   //    output value of this function at that input speed.
   // 4- Feed new input/output set into a power curve fitter. Change function
diff --git a/packages/flutter/lib/src/widgets/scroll_simulation.dart b/packages/flutter/lib/src/widgets/scroll_simulation.dart
index 2ea0814e0e..efc46dbfaf 100644
--- a/packages/flutter/lib/src/widgets/scroll_simulation.dart
+++ b/packages/flutter/lib/src/widgets/scroll_simulation.dart
@@ -51,6 +51,8 @@ class BouncingScrollSimulation extends Simulation {
       _springSimulation = _overscrollSimulation(position, velocity);
       _springTime = double.negativeInfinity;
     } else {
+      // Taken from UIScrollView.decelerationRate (.normal = 0.998)
+      // 0.998^1000 = ~0.135
       _frictionSimulation = FrictionSimulation(0.135, position, velocity);
       final double finalX = _frictionSimulation.finalX;
       if (velocity > 0.0 && finalX > trailingExtent) {
diff --git a/packages/flutter/test/widgets/scroll_physics_test.dart b/packages/flutter/test/widgets/scroll_physics_test.dart
index 18a1b9877e..fdb7a14291 100644
--- a/packages/flutter/test/widgets/scroll_physics_test.dart
+++ b/packages/flutter/test/widgets/scroll_physics_test.dart
@@ -97,6 +97,26 @@ void main() {
     );
   });
 
+  test('ScrollPhysics scrolling subclasses - Creating the simulation doesn\'t alter the velocity for time 0', () {
+    final ScrollMetrics position = FixedScrollMetrics(
+      minScrollExtent: 0.0,
+      maxScrollExtent: 100.0,
+      pixels: 20.0,
+      viewportDimension: 500.0,
+      axisDirection: AxisDirection.down,
+    );
+
+    const BouncingScrollPhysics bounce = BouncingScrollPhysics();
+    const ClampingScrollPhysics clamp = ClampingScrollPhysics();
+    const PageScrollPhysics page = PageScrollPhysics();
+
+    // Calls to createBallisticSimulation may happen on every frame (i.e. when the maxScrollExtent changes)
+    // Changing velocity for time 0 may cause a sudden, unwanted damping/speedup effect
+    expect(bounce.createBallisticSimulation(position, 1000).dx(0), moreOrLessEquals(1000));
+    expect(clamp.createBallisticSimulation(position, 1000).dx(0), moreOrLessEquals(1000));
+    expect(page.createBallisticSimulation(position, 1000).dx(0), moreOrLessEquals(1000));
+  });
+
   group('BouncingScrollPhysics test', () {
     BouncingScrollPhysics physicsUnderTest;