Fix the size of scrollbars when using the Windows Classic theme.

ui/native_theme/native_theme_win.cc

Index: ui/native_theme/native_theme_win.cc
diff --git a/ui/native_theme/native_theme_win.cc b/ui/native_theme/native_theme_win.cc
index b3d09a49d9ba0871a70f96cde7a591bee43cc2a6..564d4016683f8c76f32b4e89afaac74f3d740211 100644
--- a/ui/native_theme/native_theme_win.cc
+++ b/ui/native_theme/native_theme_win.cc
@@ -73,6 +73,10 @@ const int kSystemColors[] = {
   COLOR_MENUHIGHLIGHT,
 };
 
+const int kUnthemedScrollbarThickness =
+    static_cast<int>(GetSystemMetrics(SM_CXVSCROLL) /

[sky, 2013/02/20 22:22:47]

Can't GetSystemMetrics change?

[kevers, 2013/02/20 22:37:22]

The system metric could change if you update your theme while running chrome;
however, checking the system metric for each part size call would reintroduce
the perf regression.

A bit of background...  WebKit used to pick up the scrollbar with a single call
to GetSystemMetrics and cache the value.  The problem is that the system metrics
is in pixels and WebKit in DIP.  Thus, we got fat scrollbars in Windows when
using high-DPI mode.  This fix addresses the fat scrollbars problem, while
retaining much of the efficiency that we had earlier in the WebKit
implementation by using a cached value.  Seems like a reasonable compromise.

[sky, 2013/02/20 22:59:00]

When that happens don't we get a WM_SYSCOLORCHANGE? You should make this a
member and update it when the system colors changes.

+                     ui::win::GetDeviceScaleFactor());
+
 void SetCheckerboardShader(SkPaint* paint, const RECT& align_rect) {
   // Create a 2x2 checkerboard pattern using the 3D face and highlight colors.
   const SkColor face = color_utils::GetSysSkColor(COLOR_3DFACE);
@@ -254,6 +258,14 @@ gfx::Size NativeThemeWin::GetPartSize(Part part,
         size.cx = 13;
         size.cy = 13;
         break;
+      case kScrollbarDownArrow:
+      case kScrollbarLeftArrow:
+      case kScrollbarRightArrow:
+      case kScrollbarUpArrow:
+      case kScrollbarHorizontalThumb:
+      case kScrollbarVerticalThumb:
+        size.cx = size.cy = kUnthemedScrollbarThickness;
+        break;
       default:
         size.cx = 0;
         size.cy = 0;