Mac: Don't repaint scrollbars every frame

third_party/WebKit/Source/platform/scroll/ScrollbarTheme.h

Index: third_party/WebKit/Source/platform/scroll/ScrollbarTheme.h
diff --git a/third_party/WebKit/Source/platform/scroll/ScrollbarTheme.h b/third_party/WebKit/Source/platform/scroll/ScrollbarTheme.h
index f078c02a657b570e851d0aad5a1982ea0f4d0a91..bce24a9854f996252eff5d7321d923ada75e0775 100644
--- a/third_party/WebKit/Source/platform/scroll/ScrollbarTheme.h
+++ b/third_party/WebKit/Source/platform/scroll/ScrollbarTheme.h
@@ -38,6 +38,7 @@ class CullRect;
 class GraphicsContext;
 class PlatformMouseEvent;
 class ScrollbarThemeClient;
+class ScrollbarThemePaintParams;
 
 class PLATFORM_EXPORT ScrollbarTheme {
     WTF_MAKE_NONCOPYABLE(ScrollbarTheme); USING_FAST_MALLOC(ScrollbarTheme);
@@ -45,6 +46,13 @@ public:
     ScrollbarTheme() { }
     virtual ~ScrollbarTheme() { }
 
+    // If true, then scrollbars with this theme will be painted every time
+    // Scrollbar::setNeedsPaintInvalidation is called. If false, then scrollbar
+    // thumb and track part painting results will be cached and not repainted
+    // unless requested by Scrollbar::setNeedsPaintThumb or
+    // Scrollbar::setNeedsPaintTrack.
+    virtual bool shouldRepaintAllPartsOnInvalidation() const { return true; }
+
     virtual void updateEnabledState(const ScrollbarThemeClient*) { }
 
     virtual bool paint(const ScrollbarThemeClient*, GraphicsContext*, const CullRect&);
@@ -77,6 +85,10 @@ public:
     virtual int trackPosition(const ScrollbarThemeClient*);
     // The length of the track along the axis of the scrollbar.
     virtual int trackLength(const ScrollbarThemeClient*);
+    // The opacity of the thumb (only non-opaque for Mac overlay scrollbars).

[chrishtr, 2015/11/23 18:59:42]

Don't specify Mac-specific semantics.

[ccameron, 2015/11/24 01:12:05]

Removed (I'd prefer to say specify the use, at least with an "e.g" if not an
"only", though).

+    virtual float thumbOpacity(const ScrollbarThemeClient*) const { return 1.0f; }

[chrishtr, 2015/11/23 18:59:42]

How is the client supposed to know whether the opacity is included in the
SkPicture painted by the scrollbar, or it's ok to animate it separately?

[ccameron, 2015/11/24 01:12:05]

Opacity is always taken into account when re-drawing (it has no cost to change,
unlike re-painting the SkPicture).

[chrishtr, 2015/11/24 19:02:07]

What I mean is: what is the correct way to interpret this API from the cc
perspective? If opacity is < 1 then always apply it as an effect on top of the
SkPicture, and if opacity == 1 don't? And this works for non-Mac because there
is no transparency for the other themes right now?

+    // The opacity of the track (only non-opaque for Mac overlay scrollbars).
+    virtual float trackOpacity(const ScrollbarThemeClient*) const { return 1.0f; }
 
     virtual bool hasButtons(const ScrollbarThemeClient*) = 0;
     virtual bool hasThumb(const ScrollbarThemeClient*) = 0;