It appears that the GTK theme doesn't always draw the full rectangle

webkit/port/platform/chromium/ScrollbarThemeChromiumLinux.cpp

 // Copyright (c) 2008, Google Inc.
 // All rights reserved.
 //
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 // notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
@@ skipping 108 matching lines @@
     const GtkThemeWidgetType track_type =
         horz ? MOZ_GTK_SCROLLBAR_TRACK_HORIZONTAL : MOZ_GTK_SCROLLBAR_TRACK_VERTICAL;
     paintScrollbarWidget(gc, rect, track_type, 0);
 
     return;
 }
 
 void ScrollbarThemeChromium::paintButton(GraphicsContext* gc, Scrollbar* scrollbar,
                                     const IntRect& rect, ScrollbarPart part)
 {
+    // TODO(port): It appears the either we're upsetting GTK by forcing WebKit
+    // sizes on it, or the buttons expect the track to be drawn under them.
+    // Either way, we end up with unpainted pixels which are upsetting the
+    // pixel tests. Thus we paint green under the buttons to, at least, make
+    // the pixel output the same between debug and opt builds.
+    SkPaint paint;
+    paint.setARGB(255, 0, 255, 128);
+    SkRect skrect;
+    skrect.set(rect.x(), rect.y(), rect.x() + rect.width() - 1, rect.y() + rect.height() + 1);
+    gc->platformContext()->canvas()->drawRect(skrect, paint);
+
     const bool horz = scrollbar->orientation() == HorizontalScrollbar;
     gint flags = horz ? 0 : MOZ_GTK_STEPPER_VERTICAL;
     flags |= ForwardButtonEndPart == part ? MOZ_GTK_STEPPER_DOWN : 0;
     paintScrollbarWidget(gc, rect, MOZ_GTK_SCROLLBAR_BUTTON, flags);
 }
 
 void ScrollbarThemeChromium::paintThumb(GraphicsContext* gc, Scrollbar* scrollbar, const IntRect& rect)
 {
     const bool horz = scrollbar->orientation() == HorizontalScrollbar;
     const GtkThemeWidgetType thumb_type =
         horz ? MOZ_GTK_SCROLLBAR_THUMB_HORIZONTAL : MOZ_GTK_SCROLLBAR_THUMB_VERTICAL;
     paintScrollbarWidget(gc, rect, thumb_type, 0);
 }
 
 }  // namespace WebCore