From 05cd2e92ab8736c174ff8d9eec1ab2380764e372 Mon Sep 17 00:00:00 2001
From: Hans Muller <hansmuller@chromium.org>
Date: Tue, 14 Jul 2015 15:36:42 -0700
Subject: [PATCH] Correct bottom overscroll in VariableHeightScrollable

Also updated the createDefaultScrollSimulation()
springConstant and drag parameters to make
non-overscroll fling scrolls slowdown faster and
overscrolls snap back into place a little more
snappily.

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/1241733002 .
---
 sdk/lib/animation/scroll_behavior.dart          |  6 +++---
 sdk/lib/widgets/scrollable.dart                 |  1 +
 sdk/lib/widgets/variable_height_scrollable.dart | 15 ++++++++++++---
 3 files changed, 16 insertions(+), 6 deletions(-)

diff --git a/sdk/lib/animation/scroll_behavior.dart b/sdk/lib/animation/scroll_behavior.dart
index b69c05db0ab..b01713c888d 100644
--- a/sdk/lib/animation/scroll_behavior.dart
+++ b/sdk/lib/animation/scroll_behavior.dart
@@ -49,8 +49,8 @@ class BoundedBehavior extends ScrollBehavior {
 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);
 }
 
@@ -76,7 +76,7 @@ class OverscrollBehavior extends BoundedBehavior {
     // 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) {
diff --git a/sdk/lib/widgets/scrollable.dart b/sdk/lib/widgets/scrollable.dart
index 1195a6a9b86..6a687b5fa67 100644
--- a/sdk/lib/widgets/scrollable.dart
+++ b/sdk/lib/widgets/scrollable.dart
@@ -13,6 +13,7 @@ import 'package:sky/widgets/basic.dart';
 const double _kMillisecondsPerSecond = 1000.0;
 
 double _velocityForFlingGesture(double eventVelocity) {
+  // eventVelocity is pixels/second, config min,max limits are pixels/ms
   return eventVelocity.clamp(-config.kMaxFlingVelocity, config.kMaxFlingVelocity) /
     _kMillisecondsPerSecond;
 }
diff --git a/sdk/lib/widgets/variable_height_scrollable.dart b/sdk/lib/widgets/variable_height_scrollable.dart
index ed74d11ca23..58653aeabfa 100644
--- a/sdk/lib/widgets/variable_height_scrollable.dart
+++ b/sdk/lib/widgets/variable_height_scrollable.dart
@@ -19,9 +19,12 @@ class VariableHeightScrollable extends Scrollable {
 
   IndexedBuilder builder;
   Object token;
+  bool _contentsChanged = true;
 
   void syncFields(VariableHeightScrollable source) {
     builder = source.builder;
+    if (token != source.token)
+      _contentsChanged = true;
     token = source.token;
     super.syncFields(source);
   }
@@ -40,9 +43,15 @@ class VariableHeightScrollable extends Scrollable {
     bool didReachLastChild
   ) {
     assert(childOffsets.length > 0);
-    scrollBehavior.contentsSize = didReachLastChild ? childOffsets.last : double.INFINITY;
-    if (didReachLastChild && scrollOffset > scrollBehavior.maxScrollOffset)
-      settleScrollOffset();
+    if (didReachLastChild) {
+      scrollBehavior.contentsSize = childOffsets.last;
+      if (_contentsChanged && scrollOffset > scrollBehavior.maxScrollOffset) {
+        _contentsChanged = false;
+        settleScrollOffset();
+      }
+    } else {
+      scrollBehavior.contentsSize = double.INFINITY;
+    }
   }
 
   Widget buildContent() {