Mac: Don't repaint scrollbars every frame

third_party/WebKit/Source/platform/mac/ScrollAnimatorMac.mm

Index: third_party/WebKit/Source/platform/mac/ScrollAnimatorMac.mm
diff --git a/third_party/WebKit/Source/platform/mac/ScrollAnimatorMac.mm b/third_party/WebKit/Source/platform/mac/ScrollAnimatorMac.mm
index 4e24d784b0aec535484986268dcbf54414ba0bb4..2110b5f145d870bc772899fe066d1132c471a2cb 100644
--- a/third_party/WebKit/Source/platform/mac/ScrollAnimatorMac.mm
+++ b/third_party/WebKit/Source/platform/mac/ScrollAnimatorMac.mm
@@ -433,9 +433,11 @@ private:
         break;
     case UIStateTransition:
         [_scrollbarPainter.get() setUiStateTransitionProgress:currentValue];
+        _scrollbar->setThumbNeedsRepaint(true);
         break;
     case ExpansionTransition:
         [_scrollbarPainter.get() setExpansionTransitionProgress:currentValue];
+        _scrollbar->setThumbNeedsRepaint(true);
         break;
     }
 
@@ -460,6 +462,7 @@ private:
     RetainPtr<WebScrollbarPartAnimation> _trackAlphaAnimation;
     RetainPtr<WebScrollbarPartAnimation> _uiStateTransitionAnimation;
     RetainPtr<WebScrollbarPartAnimation> _expansionTransitionAnimation;
+    BOOL _hasExpandedSinceInvisible;
 }
 - (id)initWithScrollbar:(blink::Scrollbar*)scrollbar;
 - (void)updateVisibilityImmediately:(bool)show;
@@ -642,6 +645,20 @@ private:
 
 - (void)scrollerImp:(id)scrollerImp overlayScrollerStateChangedTo:(NSUInteger)newOverlayScrollerState
 {
+    enum {
+        NSScrollerStateInvisible = 0,

[chrishtr, 2015/11/24 21:15:12]

Forgive my ignorance of Objective C. When is NSScrollerStateInvisible ever set?

[ccameron, 2015/11/25 00:23:01]

That is a value of the newOverlayScrollerState parameter.

This (and the rest of the file) are undocumented private APIs that come from
WebKit (checked in back when this was common with Safari). The enum values of
newOverlayScrollerState are not documented -- I've found empirically that this
is called with the listed 3 states, and so I've added labels for them here.

+        NSScrollerStateKnob = 1,
+        NSScrollerStateExpanded = 2
+    };
+    // We do not receive notifications about the thumb un-expanding when the scrollbar fades away. Ensure
+    // that we re-paint the thumb the next time that we transition away from being invisible, so that
+    // the thumb doesn't stick in an expanded state.
+    if (newOverlayScrollerState == NSScrollerStateExpanded) {
+        _hasExpandedSinceInvisible = YES;
+    } else if (newOverlayScrollerState != NSScrollerStateInvisible && _hasExpandedSinceInvisible) {
+        _scrollbar->setThumbNeedsRepaint(true);
+        _hasExpandedSinceInvisible = NO;
+    }
 }
 
 - (void)invalidate
@@ -799,10 +816,6 @@ void ScrollAnimatorMac::mouseMovedInContentArea() const
 
 void ScrollAnimatorMac::mouseEnteredScrollbar(Scrollbar* scrollbar) const
 {
-    // At this time, only legacy scrollbars needs to send notifications here.
-    if (ScrollbarThemeMacCommon::recommendedScrollerStyle() != NSScrollerStyleLegacy)

[chrishtr, 2015/11/24 21:15:12]

What do the changes here and on line 819 do?

[ccameron, 2015/11/25 00:23:02]

I set the wrong upstream branch -- this is part of another commit (it's gone
now).

For reference, the comment comes in from a WebKit commit in 2012
(https://bugs.webkit.org/show_bug.cgi?id=76133), and is not true on 10.9+.
Again, no documentation, behavior changing between OS releases.

-        return;
-
     if (!scrollableArea()->scrollbarsCanBeActive())
         return;
 
@@ -816,10 +829,6 @@ void ScrollAnimatorMac::mouseEnteredScrollbar(Scrollbar* scrollbar) const
 
 void ScrollAnimatorMac::mouseExitedScrollbar(Scrollbar* scrollbar) const
 {
-    // At this time, only legacy scrollbars needs to send notifications here.
-    if (ScrollbarThemeMacCommon::recommendedScrollerStyle() != NSScrollerStyleLegacy)
-        return;
-
     if (!scrollableArea()->scrollbarsCanBeActive())
         return;
 
@@ -1077,6 +1086,8 @@ void ScrollAnimatorMac::updateScrollerStyle()
     NSScrollerStyle newStyle = [m_scrollbarPainterController.get() scrollerStyle];
 
     if (Scrollbar* verticalScrollbar = scrollableArea()->verticalScrollbar()) {
+        verticalScrollbar->setTrackBackgroundNeedsRepaint(true);
+        verticalScrollbar->setThumbNeedsRepaint(true);
         verticalScrollbar->setNeedsPaintInvalidation();
 
         ScrollbarPainter oldVerticalPainter = [m_scrollbarPainterController.get() verticalScrollerImp];
@@ -1095,6 +1106,8 @@ void ScrollAnimatorMac::updateScrollerStyle()
     }
 
     if (Scrollbar* horizontalScrollbar = scrollableArea()->horizontalScrollbar()) {
+        horizontalScrollbar->setTrackBackgroundNeedsRepaint(true);
+        horizontalScrollbar->setThumbNeedsRepaint(true);
         horizontalScrollbar->setNeedsPaintInvalidation();
 
         ScrollbarPainter oldHorizontalPainter = [m_scrollbarPainterController.get() horizontalScrollerImp];