Mac: Don't repaint scrollbars every frame

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

 /*
  * Copyright (C) 2013 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 115 matching lines @@
 }
 
 void ScrollbarThemeMacOverlayAPI::setNewPainterForScrollbar(ScrollbarThemeClient* scrollbar, ScrollbarPainter newPainter)
 {
     RetainPtr<WebCoreScrollbarObserver> observer = [[WebCoreScrollbarObserver alloc] initWithScrollbar:scrollbar painter:newPainter];
     scrollbarPainterMap()->set(scrollbar, observer);
     updateEnabledState(scrollbar);
     updateScrollbarOverlayStyle(scrollbar);
 }
 
-ScrollbarPainter ScrollbarThemeMacOverlayAPI::painterForScrollbar(const ScrollbarThemeClient* scrollbar)
+ScrollbarPainter ScrollbarThemeMacOverlayAPI::painterForScrollbar(const ScrollbarThemeClient* scrollbar) const
 {
     return [scrollbarPainterMap()->get(const_cast<ScrollbarThemeClient*>(scrollbar)).get() painter];
 }
 
 void ScrollbarThemeMacOverlayAPI::paintTrackBackground(GraphicsContext* context, const ScrollbarThemeClient* scrollbar, const IntRect& rect) {
     if (DrawingRecorder::useCachedDrawingIfPossible(*context, *scrollbar, DisplayItem::ScrollbarTrackBackground))
         return;
 
     DrawingRecorder recorder(*context, *scrollbar, DisplayItem::ScrollbarTrackBackground, rect);
 
     ASSERT(isOverlayAPIAvailable());
 
     GraphicsContextStateSaver stateSaver(*context);
     context->translate(rect.x(), rect.y());
     LocalCurrentGraphicsContext localContext(context, IntRect(IntPoint(), rect.size()));
 
     CGRect frameRect = scrollbar->frameRect();
     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];
 }
 
 void ScrollbarThemeMacOverlayAPI::paintThumb(GraphicsContext* context, const ScrollbarThemeClient* scrollbar, const IntRect& rect) {
     if (DrawingRecorder::useCachedDrawingIfPossible(*context, *scrollbar, DisplayItem::ScrollbarThumb))
         return;
 
     // Expand dirty rect to allow for scroll thumb anti-aliasing in minimum thumb size case.
     IntRect dirtyRect = IntRect(rect);
     dirtyRect.inflate(1);
     DrawingRecorder recorder(*context, *scrollbar, DisplayItem::ScrollbarThumb, dirtyRect);
 
     ASSERT(isOverlayAPIAvailable());
 
     GraphicsContextStateSaver stateSaver(*context);
     context->translate(rect.x(), rect.y());
     LocalCurrentGraphicsContext localContext(context, IntRect(IntPoint(), rect.size()));
 
     ScrollbarPainter scrollbarPainter = painterForScrollbar(scrollbar);
     [scrollbarPainter setEnabled:scrollbar->enabled()];
     [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)
 {
     NSControlSize nsControlSize = static_cast<NSControlSize>(controlSize);
     ScrollbarPainter scrollbarPainter = [NSClassFromString(@"NSScrollerImp") scrollerImpWithStyle:recommendedScrollerStyle() controlSize:nsControlSize horizontal:NO replacingScrollerImp:nil];
     BOOL wasExpanded = NO;
     if (supportsExpandedScrollbars()) {
         wasExpanded = [scrollbarPainter isExpanded];
         [scrollbarPainter setExpanded:YES];
@@ skipping 61 matching lines @@
 int ScrollbarThemeMacOverlayAPI::minimumThumbLength(const ScrollbarThemeClient* scrollbar)
 {
     return [painterForScrollbar(scrollbar) knobMinLength];
 }
 
 void ScrollbarThemeMacOverlayAPI::updateEnabledState(const ScrollbarThemeClient* scrollbar)
 {
     [painterForScrollbar(scrollbar) setEnabled:scrollbar->enabled()];
 }
 
+float ScrollbarThemeMacOverlayAPI::thumbOpacity(const ScrollbarThemeClient* scrollbar) const {
+    ScrollbarPainter scrollbarPainter = painterForScrollbar(scrollbar);
+    return [scrollbarPainter knobAlpha];
+}
+
 } // namespace blink