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 kOverlayStrokeAlphaNormal = 0x4D;

[Evan Stade, 2016/12/15 22:02:49]

nit: the naming on these constants is a little inconsistent. It would be more
symmetric if it were OverlayScrollbarFillAlpha vs OverlayScrollbarStrokeAlpha.
That said, I would be in favor of inlining all these since they're only used
once, and getting rid of the ones that don't differ between stroke and fill.

[bokan, 2016/12/15 22:05:30]

Agreed on naming. Personally, I like having named constants since it makes it
easy to see where the style values are coming from and where to change but a
middle ground might be to move them into the scope where they're used.

+constexpr SkAlpha kOverlayStrokeAlphaHovered = 0x58;
+constexpr SkAlpha kOverlayStrokeAlphaPressed = 0x80;
 
 // 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 140 matching lines @@
     State state,
     const gfx::Rect& rect,
     ScrollbarOverlayColorTheme theme) const {
   // Do not paint if state is disabled.
   if (state == kDisabled)
     return;
 
   TRACE_EVENT0("blink", "NativeThemeAura::PaintScrollbarThumb");
 
   SkAlpha thumb_alpha = SK_AlphaTRANSPARENT;
+  SkAlpha stroke_alpha = SK_AlphaTRANSPARENT;
   const bool overlay = use_overlay_scrollbars_;
   switch (state) {
     case NativeTheme::kDisabled:
       thumb_alpha = SK_AlphaTRANSPARENT;
+      stroke_alpha = SK_AlphaTRANSPARENT;
       break;
     case NativeTheme::kHovered:
       thumb_alpha = overlay ? kOverlayScrollbarAlphaHovered : 0x4D;
+      stroke_alpha = overlay ? kOverlayStrokeAlphaHovered : 0x4D;
       break;
     case NativeTheme::kNormal:
       thumb_alpha = overlay ? kOverlayScrollbarAlphaNormal : 0x33;
+      stroke_alpha = overlay ? kOverlayStrokeAlphaNormal : 0x33;
       break;
     case NativeTheme::kPressed:
       thumb_alpha = overlay ? kOverlayScrollbarAlphaPressed : 0x80;
+      stroke_alpha = overlay ? kOverlayStrokeAlphaPressed : 0x80;

[Evan Stade, 2016/12/15 22:02:49]

stroke_alpha is unused when !overlay

       break;
     case NativeTheme::kNumStates:
       NOTREACHED();
       break;
   }
 
   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;
     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);

[Evan Stade, 2016/12/15 22:02:49]

nit: you can do Inset(gfx::Insets(kStrokeWidth));

(also L240)

   } else {
     // If there are no scrollbuttons then provide some padding so that the thumb
     // doesn't touch the top of the track.
     const int kThumbPadding = 2;
     const int extra_padding =
         (scrollbar_button_length() == 0) ? kThumbPadding : 0;
     if (part == NativeTheme::kScrollbarVerticalThumb)
       thumb_rect.Inset(kThumbPadding, extra_padding);
     else
       thumb_rect.Inset(extra_padding, kThumbPadding);
@@ skipping 35 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