Overlay scrollbars flush with window edge

ui/native_theme/native_theme_aura.cc

Index: ui/native_theme/native_theme_aura.cc
diff --git a/ui/native_theme/native_theme_aura.cc b/ui/native_theme/native_theme_aura.cc
index 76283878e06a13f8cb77b8adbc90799dfea2aa28..1c301c8bf70bf32e65e650097b1dbfa60991391e 100644
--- a/ui/native_theme/native_theme_aura.cc
+++ b/ui/native_theme/native_theme_aura.cc
@@ -186,7 +186,7 @@ void NativeThemeAura::PaintScrollbarThumb(
     Part part,
     State state,
     const gfx::Rect& rect,
-    ScrollbarOverlayColorTheme theme) const {
+    const NativeTheme::ScrollbarThumbExtraParams& params) const {
   // Do not paint if state is disabled.
   if (state == kDisabled)
     return;
@@ -212,7 +212,7 @@ void NativeThemeAura::PaintScrollbarThumb(
     constexpr SkColor kOverlayScrollbarStrokeColor[] = {SK_ColorWHITE,
                                                         SK_ColorBLACK};
 
-    thumb_color = kOverlayScrollbarThumbColor[theme];
+    thumb_color = kOverlayScrollbarThumbColor[params.scrollbar_theme];
 
     SkAlpha stroke_alpha = SK_AlphaTRANSPARENT;
     switch (state) {
@@ -240,18 +240,33 @@ void NativeThemeAura::PaintScrollbarThumb(
     // In overlay mode, draw a stroke (border).
     constexpr int kStrokeWidth = kOverlayScrollbarStrokeWidth;
     cc::PaintFlags flags;
-    flags.setColor(
-        SkColorSetA(kOverlayScrollbarStrokeColor[theme], stroke_alpha));
+    flags.setColor(SkColorSetA(
+        kOverlayScrollbarStrokeColor[params.scrollbar_theme], stroke_alpha));
     flags.setStyle(cc::PaintFlags::kStroke_Style);
     flags.setStrokeWidth(kStrokeWidth);
 
+    int leftStrokeWidth, topStrokeWidth, rightStrokeWidth, bottomStrokeWidth;
+    leftStrokeWidth = topStrokeWidth = rightStrokeWidth = bottomStrokeWidth =
+        kStrokeWidth;
+
+    // Flush the stroke when it close to border.

[bokan, 2017/03/23 21:34:41]

A better comment would be "The edge to which the scrollbar is attached shouldn't
have a border"

+    if (part == NativeTheme::kScrollbarVerticalThumb) {
+      if (params.isLeftVerticalScrollbar)
+        leftStrokeWidth = 0;
+      else
+        rightStrokeWidth = 0;
+    } else {
+      bottomStrokeWidth = 0;
+    }
+
     gfx::RectF stroke_rect(thumb_rect);
-    constexpr float kHalfStrokeWidth = kStrokeWidth / 2.f;
-    stroke_rect.Inset(kHalfStrokeWidth, kHalfStrokeWidth);
+    stroke_rect.Inset(leftStrokeWidth / 2.f, topStrokeWidth / 2.f,

[bokan, 2017/03/23 21:34:41]

The drawRect call below is the one that draws the outline. Since we're going to
paint over it with the fill anyway, there's no need to change the inset above
(in fact I think this moves the stroke out a little). You should only change the
inset after the drawRect call below so that the fill paints over that one side.
(as I did in my patch).

I also think it'd be clearer to just put the thumb_rect.Inset call itself inside
the if statements and vary which parameter gets the 0 based on
vertical|horizontal|isLeftVertical, rather than define left, right, top, bottom
stroke widths.

+                      rightStrokeWidth / 2.f, bottomStrokeWidth / 2.f);
     canvas->drawRect(gfx::RectFToSkRect(stroke_rect), flags);
 
     // Inset the all the edges edges so we fill-in the stroke below.
-    thumb_rect.Inset(kStrokeWidth, kStrokeWidth);
+    thumb_rect.Inset(leftStrokeWidth, topStrokeWidth, rightStrokeWidth,
+                     bottomStrokeWidth);
   } else {
     switch (state) {
       case NativeTheme::kDisabled: