Mac: Don't repaint scrollbars every frame

third_party/WebKit/Source/platform/scroll/ScrollbarThemeMacOverlayAPI.mm

Index: third_party/WebKit/Source/platform/scroll/ScrollbarThemeMacOverlayAPI.mm
diff --git a/third_party/WebKit/Source/platform/scroll/ScrollbarThemeMacOverlayAPI.mm b/third_party/WebKit/Source/platform/scroll/ScrollbarThemeMacOverlayAPI.mm
index eed11375dcfe60a43303a211b821ba03238554b6..91d5131b11a9dee30de83fbc5ad6940b13562e22 100644
--- a/third_party/WebKit/Source/platform/scroll/ScrollbarThemeMacOverlayAPI.mm
+++ b/third_party/WebKit/Source/platform/scroll/ScrollbarThemeMacOverlayAPI.mm
@@ -133,7 +133,7 @@ void ScrollbarThemeMacOverlayAPI::setNewPainterForScrollbar(ScrollbarThemeClient
     updateScrollbarOverlayStyle(scrollbar);
 }
 
-ScrollbarPainter ScrollbarThemeMacOverlayAPI::painterForScrollbar(const ScrollbarThemeClient* scrollbar)
+ScrollbarPainter ScrollbarThemeMacOverlayAPI::painterForScrollbar(const ScrollbarThemeClient* scrollbar) const
 {
     return [scrollbarPainterMap()->get(const_cast<ScrollbarThemeClient*>(scrollbar)).get() painter];
 }
@@ -154,7 +154,6 @@ void ScrollbarThemeMacOverlayAPI::paintTrackBackground(GraphicsContext* context,
     ScrollbarPainter scrollbarPainter = painterForScrollbar(scrollbar);
     [scrollbarPainter setEnabled:scrollbar->enabled()];
     [scrollbarPainter setBoundsSize: NSSizeFromCGSize(frameRect.size)];
-
     NSRect trackRect = NSMakeRect(0, 0, frameRect.size.width, frameRect.size.height);
     [scrollbarPainter drawKnobSlotInRect:trackRect highlight:NO];
 }
@@ -179,12 +178,17 @@ void ScrollbarThemeMacOverlayAPI::paintThumb(GraphicsContext* context, const Scr
     [scrollbarPainter setBoundsSize:NSSizeFromCGSize(rect.size())];
     [scrollbarPainter setDoubleValue:0];
     [scrollbarPainter setKnobProportion:1];
+
+    CGFloat oldKnobAlpha = [scrollbarPainter knobAlpha];
+    [scrollbarPainter setKnobAlpha:1];
+
     if (scrollbar->enabled())
         [scrollbarPainter drawKnob];
 
     // If this state is not set, then moving the cursor over the scrollbar area will only cause the
     // scrollbar to engorge when moved over the top of the scrollbar area.
     [scrollbarPainter setBoundsSize: NSSizeFromCGSize(scrollbar->frameRect().size())];
+    [scrollbarPainter setKnobAlpha:oldKnobAlpha];
 }
 
 int ScrollbarThemeMacOverlayAPI::scrollbarThickness(ScrollbarControlSize controlSize)
@@ -266,5 +270,10 @@ void ScrollbarThemeMacOverlayAPI::updateEnabledState(const ScrollbarThemeClient*
     [painterForScrollbar(scrollbar) setEnabled:scrollbar->enabled()];
 }
 
+float ScrollbarThemeMacOverlayAPI::thumbOpacity(const ScrollbarThemeClient* scrollbar) const {
+    ScrollbarPainter scrollbarPainter = painterForScrollbar(scrollbar);
+    return [scrollbarPainter knobAlpha];
+}
+
 } // namespace blink