Overlay scrollbars flush with window edge

ui/native_theme/native_theme.h

 // Copyright (c) 2012 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.
 
 #ifndef UI_NATIVE_THEME_NATIVE_THEME_H_
 #define UI_NATIVE_THEME_NATIVE_THEME_H_
 
 #include "base/macros.h"
 #include "base/observer_list.h"
 #include "build/build_config.h"
@@ skipping 184 matching lines @@
   };
 
   enum ScrollbarOverlayColorTheme {
     ScrollbarOverlayColorThemeDark,
     ScrollbarOverlayColorThemeLight
   };
 
   struct ScrollbarThumbExtraParams {
     bool is_hovering;
     ScrollbarOverlayColorTheme scrollbar_theme;
+    bool isVerticalScrollbar;

[bokan, 2017/03/22 20:07:25]

You shouldn't need to pass this param from Blink, PaintScrollbarThumb is called
form NativeThemeBase::paint with either kScrollbarHorizontalThumb or
kScrollbarVerticalThumb so you can decide there.

+    bool isLeftVerticalScrollbar;
+    bool hitStart;

[bokan, 2017/03/22 20:07:25]

We should avoid repainting. A simpler solution would be to expand the minimum
and maximum positions of the track by 1 pixel. i.e. the track should start at
-1. Have you tried that?

It looks like we already have something like this in the scrollbar "margin"
though currently in Blink that appears to be used for both dimensions, you'd
need to split it up into separate margins for scroll direction and
perpendicular. On the compositor that gets plumbed through as TrackStart() which
seems like what we're looking for, you'd just need to set a -1 margin. 

+    bool hitEnd;
   };
 
   struct SliderExtraParams {
     bool vertical;
     bool in_drag;
   };
 
   struct TextFieldExtraParams {
     bool is_text_area;
     bool is_listbox;
@@ skipping 216 matching lines @@
  private:
   // Observers to notify when the native theme changes.
   base::ObserverList<NativeThemeObserver> native_theme_observers_;
 
   DISALLOW_COPY_AND_ASSIGN(NativeTheme);
 };
 
 }  // namespace ui
 
 #endif  // UI_NATIVE_THEME_NATIVE_THEME_H_