android scroller: Stop fling earlier.

ui/gfx/android/scroller.cc

 // Copyright 2014 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.
 
 #include "ui/gfx/android/scroller.h"
 
 #include <cmath>
 
 #include "base/lazy_instance.h"
 
 namespace gfx {
 namespace {
 
 // Default scroll duration from android.widget.Scroller.
 const int kDefaultDurationMs = 250;
 
 // Default friction constant in android.view.ViewConfiguration.
 const float kDefaultFriction = 0.015f;
 
 // == std::log(0.78f) / std::log(0.9f)
 const float kDecelerationRate = 2.3582018f;
 
 // Tension lines cross at (kInflexion, 1).
 const float kInflexion = 0.35f;
 
 const float kEpsilon = 1e-5f;
 
+// Fling scroll is stopped when the scroll position is |kThresholdForFlingEnd|
+// pixels or closer from the end.
+const float kThresholdForFlingEnd = 0.1;
+
 bool ApproxEquals(float a, float b) {
   return std::abs(a - b) < kEpsilon;
 }
 
 struct ViscosityConstants {
   ViscosityConstants()
       : viscous_fluid_scale_(8.f), viscous_fluid_normalize_(1.f) {
     viscous_fluid_normalize_ = 1.0f / ApplyViscosity(1.0f);
   }
 
@@ skipping 279 matching lines @@
           t, &distance_coef, &velocity_coef);
 
       curr_velocity_ = velocity_coef * distance_ * duration_seconds_reciprocal_;
 
       curr_x_ = start_x_ + distance_coef * delta_x_;
       curr_x_ = Clamped(curr_x_, min_x_, max_x_);
 
       curr_y_ = start_y_ + distance_coef * delta_y_;
       curr_y_ = Clamped(curr_y_, min_y_, max_y_);
 
-      if (ApproxEquals(curr_x_, final_x_) && ApproxEquals(curr_y_, final_y_)) {
+      float diff_x = std::abs(curr_x_ - final_x_);
+      float diff_y = std::abs(curr_y_ - final_y_);
+      if (diff_x < kThresholdForFlingEnd && diff_y < kThresholdForFlingEnd)
         finished_ = true;
-      }
     } break;
   }
 
   return true;
 }
 
 void Scroller::ExtendDuration(base::TimeDelta extend) {
   base::TimeDelta passed = GetTimePassed();
   duration_ = passed + extend;
   duration_seconds_reciprocal_ = 1.0 / duration_.InSecondsF();
@@ skipping 90 matching lines @@
 }
 
 double Scroller::GetSplineFlingDistance(float velocity) const {
   const double l = GetSplineDeceleration(velocity);
   const double decel_minus_one = kDecelerationRate - 1.0;
   return fling_friction_ * tuning_coeff_ *
          std::exp(kDecelerationRate / decel_minus_one * l);
 }
 
 }  // namespace gfx