Ignore rounding error between clip_layer_length_ and scroll_layer_length_

cc/layers/scrollbar_layer_impl_base.cc

 // Copyright 2013 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/scrollbar_layer_impl_base.h"
 
 #include <algorithm>
 #include "cc/trees/effect_node.h"
 #include "cc/trees/layer_tree_impl.h"
 #include "ui/gfx/geometry/rect_conversions.h"
@@ skipping 52 matching lines @@
     return false;
   current_pos_ = current_pos;
   NoteLayerPropertyChanged();
   return true;
 }
 
 bool ScrollbarLayerImplBase::CanScrollOrientation() const {
   LayerImpl* scroll_layer = layer_tree_impl()->LayerById(scroll_layer_id_);
   if (!scroll_layer)
     return false;
+
+  float page_scale_factor = layer_tree_impl()->current_page_scale_factor();
+  float device_scale_factor = layer_tree_impl()->device_scale_factor();
+
+  int clip_device_pixels =
+      std::ceil(clip_layer_length_ * page_scale_factor * device_scale_factor);
+  int scroll_device_pixels =
+      std::ceil(scroll_layer_length_ * page_scale_factor * device_scale_factor);

[bokan, 2017/04/20 22:09:49]

We should floor scroll instead of ceil, see previous comment for why.

E.g.

DSF = 1
PSF = 2.3804
Clip = 49.99
Scroll = 50

So in DP:

Clip = 118.996196
Scroll = 119.02

So if we ceil both we'll get 119 < 120 which we clearly didn't intend.

+
   return scroll_layer->user_scrollable(orientation()) &&
-         clip_layer_length_ < scroll_layer_length_;
+         clip_device_pixels < scroll_device_pixels;
 }
 
 bool ScrollbarLayerImplBase::SetVerticalAdjust(float vertical_adjust) {
   if (vertical_adjust_ == vertical_adjust)
     return false;
   vertical_adjust_ = vertical_adjust;
   NoteLayerPropertyChanged();
   return true;
 }
 
@@ skipping 145 matching lines @@
   layer_tree_impl()->AddToOpacityAnimationsMap(id(), opacity);
 
   node->opacity = opacity;
   node->effect_changed = true;
   property_trees->changed = true;
   property_trees->effect_tree.set_needs_update(true);
   layer_tree_impl()->set_needs_update_draw_properties();
 }
 
 }  // namespace cc