Alpha for overlay scrollbars decreased when hovered.

ui/native_theme/native_theme_aura.cc

 // 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.
 
 #include "ui/native_theme/native_theme_aura.h"
 
 #include <limits>
 #include <utility>
 
 #include "base/logging.h"
@@ skipping 17 matching lines @@
 
 namespace {
 
 // Constants for painting overlay scrollbars. Other properties needed outside
 // this painting code are defined in overlay_scrollbar_constants_aura.h.
 constexpr int kOverlayScrollbarStrokeWidth = 1;
 constexpr int kOverlayScrollbarMinimumLength = 12;
 constexpr SkAlpha kOverlayScrollbarAlphaNormal = 0x4D;
 constexpr SkAlpha kOverlayScrollbarAlphaHovered = 0x80;
 constexpr SkAlpha kOverlayScrollbarAlphaPressed = 0x80;
+constexpr SkAlpha kOverlayStrokeAlphaHovered = 0x58;
 
 // Indexed by ScrollbarOverlayColorTheme.
 constexpr SkColor kOverlayScrollbarThumbColor[] = {SK_ColorBLACK,
                                                    SK_ColorWHITE};
 constexpr SkColor kOverlayScrollbarStrokeColor[] = {SK_ColorWHITE,
                                                     SK_ColorBLACK};
 
 const SkColor kTrackColor = SkColorSetRGB(0xF1, 0xF1, 0xF1);
 
 }  // namespace
@@ skipping 167 matching lines @@
   }
 
   gfx::Rect thumb_rect(rect);
   SkColor thumb_color;
   if (overlay) {
     thumb_color = kOverlayScrollbarThumbColor[theme];
 
     // In overlay mode, draw a stroke (border).
     constexpr int kStrokeWidth = kOverlayScrollbarStrokeWidth;
     SkPaint paint;
+
+    SkAlpha stroke_alpha = state == NativeTheme::kHovered
+                               ? kOverlayStrokeAlphaHovered
+                               : thumb_alpha;

[bokan, 2016/12/15 17:59:28]

for consistency, replace thumb_alpha here with a new kOverlayStrokeAlphaNormal

     paint.setColor(
-        SkColorSetA(kOverlayScrollbarStrokeColor[theme], thumb_alpha));
+        SkColorSetA(kOverlayScrollbarStrokeColor[theme], stroke_alpha));
     paint.setStyle(SkPaint::kStroke_Style);
     paint.setStrokeWidth(kStrokeWidth);
 
     gfx::RectF stroke_rect(thumb_rect);
     constexpr float kHalfStrokeWidth = kStrokeWidth / 2.f;
     stroke_rect.Inset(kHalfStrokeWidth, kHalfStrokeWidth);
     canvas->drawRect(gfx::RectFToSkRect(stroke_rect), paint);
 
     // Inset the all the edges edges so we fill-in the stroke below.
     thumb_rect.Inset(kStrokeWidth, kStrokeWidth);
@@ skipping 45 matching lines @@
         // scrollbars isn't asking for part sizes that don't exist.
         // crbug.com/657159.
         break;
     }
   }
 
   return NativeThemeBase::GetPartSize(part, state, extra);
 }
 
 }  // namespace ui