[Android] Suppress overscroll for unscrollable axes in the browser

cc/layers/viewport.cc

 // 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.
 
 #include "cc/layers/viewport.h"
 
 #include "base/logging.h"
 #include "cc/input/top_controls_manager.h"
 #include "cc/trees/layer_tree_host_impl.h"
 #include "cc/trees/layer_tree_impl.h"
@@ skipping 140 matching lines @@
   return false;
 }
 
 gfx::Vector2dF Viewport::AdjustOverscroll(const gfx::Vector2dF& delta) const {
   const float kEpsilon = 0.1f;
   gfx::Vector2dF adjusted = delta;
 
   if (std::abs(adjusted.x()) < kEpsilon)
     adjusted.set_x(0.0f);
   if (std::abs(adjusted.y()) < kEpsilon)
     adjusted.set_y(0.0f);

[tdresser, 2015/07/10 18:41:30]

Why can't these be moved?

[jdduke (slow), 2015/07/10 19:12:50]

So, the problem is that we're often seeing small residual overscroll deltas
after nearly ever scroll (~.01 or less) even when the "full" scroll delta has
been consume. Ideally we'd fix that, but it seems to also happen on the Blink
thread so maybe there's something more fundamental. In any case, we don't want
to report overscroll when we've logically consumed the entire scroll delta.

 
-  // Disable overscroll on axes which are impossible to scroll.
-  if (host_impl_->settings().report_overscroll_only_for_scrollable_axes) {
-    if (std::abs(MaxTotalScrollOffset().x()) <= kEpsilon ||
-        !InnerScrollLayer()->user_scrollable_horizontal())
-      adjusted.set_x(0.0f);
-    if (std::abs(MaxTotalScrollOffset().y()) <= kEpsilon ||
-        !InnerScrollLayer()->user_scrollable_vertical())
-      adjusted.set_y(0.0f);
-  }
-
   return adjusted;
 }
 
 gfx::ScrollOffset Viewport::MaxTotalScrollOffset() const {
   gfx::ScrollOffset offset;
 
   offset += InnerScrollLayer()->MaxScrollOffset();
 
   if (OuterScrollLayer())
     offset += OuterScrollLayer()->MaxScrollOffset();
@@ skipping 14 matching lines @@
 
 LayerImpl* Viewport::InnerScrollLayer() const {
   return host_impl_->InnerViewportScrollLayer();
 }
 
 LayerImpl* Viewport::OuterScrollLayer() const {
   return host_impl_->OuterViewportScrollLayer();
 }
 
 }  // namespace cc