Overlay scrollbars expand only when mouse is near thumb

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 89 matching lines @@
 }
 
 bool ScrollbarLayerImplBase::SetThumbThicknessScaleFactor(float factor) {
   if (thumb_thickness_scale_factor_ == factor)
     return false;
   thumb_thickness_scale_factor_ = factor;
   NoteLayerPropertyChanged();
   return true;
 }
 
+gfx::Rect ScrollbarLayerImplBase::ComputeExpandedThumbQuadRect() const {
+  DCHECK(is_overlay_scrollbar());
+  float track_length = TrackLength();
+  int thumb_length = ThumbLength();
+  int thumb_thickness = ThumbThickness();
+  float maximum = scroll_layer_length_ - clip_layer_length_;
+
+  // With the length known, we can compute the thumb's position.
+  float clamped_current_pos = std::min(std::max(current_pos_, 0.f), maximum);
+
+  int thumb_offset = TrackStart();
+  if (maximum > 0) {
+    float ratio = clamped_current_pos / maximum;
+    float max_offset = track_length - thumb_length;
+    thumb_offset += static_cast<int>(ratio * max_offset);
+  }
+
+  gfx::RectF thumb_rect;
+  if (orientation_ == HORIZONTAL) {
+    thumb_rect = gfx::RectF(thumb_offset, vertical_adjust_, thumb_length,
+                            thumb_thickness);
+  } else {
+    thumb_rect = gfx::RectF(is_left_side_vertical_scrollbar_
+                                ? bounds().width() - thumb_thickness
+                                : 0,
+                            thumb_offset, thumb_thickness, thumb_length);
+  }
+
+  return gfx::ToEnclosingRect(thumb_rect);
+}
+
 gfx::Rect ScrollbarLayerImplBase::ComputeThumbQuadRect() const {
   // Thumb extent is the length of the thumb in the scrolling direction, thumb
   // thickness is in the perpendicular direction. Here's an example of a
   // horizontal scrollbar - inputs are above the scrollbar, computed values
   // below:
   //
   //    |<------------------- track_length_ ------------------->|
   //
   // |--| <-- start_offset
   //
@@ skipping 57 matching lines @@
   // With the length known, we can compute the thumb's position.
   float clamped_current_pos = std::min(std::max(current_pos_, 0.f), maximum);
 
   int thumb_offset = TrackStart();
   if (maximum > 0) {
     float ratio = clamped_current_pos / maximum;
     float max_offset = track_length - thumb_length;
     thumb_offset += static_cast<int>(ratio * max_offset);
   }
 
   float thumb_thickness_adjustment =

[bokan, 2017/04/25 19:36:21]

This method should just call the new one above and apply the
thumb_thickness_adjustment to the rect returned from there.

       thumb_thickness * (1.f - thumb_thickness_scale_factor_);
 
   gfx::RectF thumb_rect;
   if (orientation_ == HORIZONTAL) {
     thumb_rect = gfx::RectF(thumb_offset,
                             vertical_adjust_ + thumb_thickness_adjustment,
                             thumb_length,
                             thumb_thickness - thumb_thickness_adjustment);
   } else {
     thumb_rect = gfx::RectF(
@@ skipping 33 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