Correct bottom overscroll in VariableHeightScrollable

sky/sdk/lib/animation/scroll_behavior.dart

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 import 'dart:math' as math;
 
 import 'package:newton/newton.dart';
 
 const double _kSecondsPerMillisecond = 1000.0;
 
@@ skipping 31 matching lines @@
   double get maxScrollOffset => math.max(0.0, _contentsSize - _containerSize);
 
   double applyCurve(double scrollOffset, double scrollDelta) {
     return (scrollOffset + scrollDelta).clamp(0.0, maxScrollOffset);
   }
 }
 
 Simulation createDefaultScrollSimulation(double position, double velocity, double minScrollOffset, double maxScrollOffset) {
   double velocityPerSecond = velocity * _kSecondsPerMillisecond;
   SpringDescription spring = new SpringDescription.withDampingRatio(
-      mass: 1.0, springConstant: 85.0, ratio: 1.1);
-  double drag = 0.4;
+      mass: 1.0, springConstant: 170.0, ratio: 1.1);
+  double drag = 0.025;
   return new ScrollSimulation(position, velocityPerSecond, minScrollOffset, maxScrollOffset, spring, drag);
 }
 
 class FlingBehavior extends BoundedBehavior {
   FlingBehavior({ double contentsSize: 0.0, double containerSize: 0.0 })
     : super(contentsSize: contentsSize, containerSize: containerSize);
 
   Simulation release(double position, double velocity) {
     return createDefaultScrollSimulation(position, 0.0, minScrollOffset, maxScrollOffset);
   }
 }
 
 class OverscrollBehavior extends BoundedBehavior {
   OverscrollBehavior({ double contentsSize: 0.0, double containerSize: 0.0 })
     : super(contentsSize: contentsSize, containerSize: containerSize);
 
   Simulation release(double position, double velocity) {
     return createDefaultScrollSimulation(position, velocity, minScrollOffset, maxScrollOffset);
   }
 
   double applyCurve(double scrollOffset, double scrollDelta) {
     double newScrollOffset = scrollOffset + scrollDelta;
     // If we're overscrolling, we want move the scroll offset 2x
     // slower than we would otherwise. Therefore, we "rewind" the
     // newScrollOffset by half the amount that we moved it above.
-    // Notice that we clap the "old" value to 0.0 so that we only
+    // Notice that we clamp the "old" value to 0.0 so that we only
     // reduce the portion of scrollDelta that's applied beyond 0.0. We
     // do similar things for overscroll in the other direction.
     if (newScrollOffset < 0.0) {
       newScrollOffset -= (newScrollOffset - math.min(0.0, scrollOffset)) / 2.0;
     } else if (newScrollOffset > maxScrollOffset) {
       newScrollOffset -= (newScrollOffset - math.max(maxScrollOffset, scrollOffset)) / 2.0;
     }
     return newScrollOffset;
   }
 }