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..c9b5403cc27efdc517820d55f3b09038bdc3159f 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) {
@@ -240,8 +240,8 @@ 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);
 
@@ -251,7 +251,15 @@ void NativeThemeAura::PaintScrollbarThumb(
     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);
+    // The edge to which the scrollbar is attached shouldn't have a border.
+    if (part == NativeTheme::kScrollbarVerticalThumb) {

[Evan Stade, 2017/03/28 00:01:56]

nit: this can be clearer/less nested, e.g.

gfx::Insets insets(kStrokeWidth);
if (part == Horizontal)
  insets -= gfx::Insets(0,0,kStrokeWidth,0);
else if (isLeftVertical)
  insets -= gfx::Insets(0,kStrokeWidth,0,0);
else
  insets -= gfx::Insets(0,0,0,kStrokeWidth);
thumb_rect.Inset(insets);

+      if (params.isLeftVerticalScrollbar)
+        thumb_rect.Inset(0, kStrokeWidth, kStrokeWidth, kStrokeWidth);
+      else
+        thumb_rect.Inset(kStrokeWidth, kStrokeWidth, 0, kStrokeWidth);
+    } else {
+      thumb_rect.Inset(kStrokeWidth, kStrokeWidth, kStrokeWidth, 0);
+    }
   } else {
     switch (state) {
       case NativeTheme::kDisabled: