(causing scroll bars not to appear/update quickly in some cases)
https://bugs.webkit.org/show_bug.cgi?id=122585
-and corresponding-
<rdar://problem/10710775>

Reviewed by Simon Fraser.

Source/WebCore: 

This patch does a few things in order to allow scrollbars to be updated on the 
scrolling thread:

1. This patch adds the ability to know if the lower-level APIs necessary to get 
this to work right are available, AND if the content is actually capable of taking 
advantage of this feature. This is currently implemented as 
Scrollbar::supportsUpdateOnSecondaryThread() which makes use of a new 
ScrollableArea function called updatesScrollLayerPositionOnMainThread()
        
2. To update on the scrolling thread, the scrolling tree needs to know about the 
ScrollbarPainters.
        
3. Once it knows about them, it should update the presentation value whenever the 
layer position changes.
        
4. Presentation value is basically the same thing as double value. There is a bit 
of code we maintain currently to compute that. This patch moves that code to a 
static function on ScrollableArea that can be called from both the main thread and 
the scrolling thread.
        
5. ScrollbarPainter API needs to know about the layers we have created for the 
vertical and horizontal scrollbars, then they will use those layers and the 
presentation value that we set on the scrolling thread to move the layers around.

This is part of #1 above. 
* page/FrameView.cpp:
(WebCore::FrameView::updatesScrollLayerPositionOnMainThread):
* page/FrameView.h:

This is part of #2. ScrollingStateScrollingNodes now have vertical and horizontal 
ScrollbarPainters for Mac only.
* page/scrolling/ScrollingStateScrollingNode.cpp:
(WebCore::ScrollingStateScrollingNode::ScrollingStateScrollingNode):
* page/scrolling/ScrollingStateScrollingNode.h:
(WebCore::ScrollingStateScrollingNode::verticalScrollbarPainter):
(WebCore::ScrollingStateScrollingNode::horizontalScrollbarPainter):

Also part of #2. Make sure to set the ScrollbarPainters for scrolling nodes when 
appropriate.
* page/scrolling/mac/ScrollingCoordinatorMac.h:
* page/scrolling/mac/ScrollingCoordinatorMac.mm:
(WebCore::ScrollingCoordinatorMac::frameViewLayoutUpdated):

Implement this function that was just stubbed out before. This is part of #5 in 
that is will allow the ScrollbarPainter API to know about any layer changes. 
(WebCore::ScrollingCoordinatorMac::scrollableAreaScrollbarLayerDidChange):

Back to #2, making sure we properly set the ScrollbarPainters to send over to the 
scrolling thread.
(WebCore::ScrollingCoordinatorMac::setScrollbarPaintersForNode):
* page/scrolling/mac/ScrollingStateScrollingNodeMac.mm:
(WebCore::ScrollingStateScrollingNode::setScrollbarPainters):

This code achieves #3. It uses new ScrollbarPainter API to adjust the position of 
the scrollbars from the scrolling thread.
* page/scrolling/mac/ScrollingTreeScrollingNodeMac.h:
* page/scrolling/mac/ScrollingTreeScrollingNodeMac.mm:
(WebCore::ScrollingTreeScrollingNodeMac::ScrollingTreeScrollingNodeMac):
(WebCore::ScrollingTreeScrollingNodeMac::updateBeforeChildren):
(WebCore::ScrollingTreeScrollingNodeMac::setScrollLayerPosition):

This is for #5. ScrollbarPainter needs to know about our scrollbar layers.
* platform/ScrollAnimator.h:
(WebCore::ScrollAnimator::verticalScrollbarLayerDidChange):
(WebCore::ScrollAnimator::horizontalScrollbarLayerDidChange):
* platform/ScrollableArea.cpp:
(WebCore::ScrollableArea::verticalScrollbarLayerDidChange):
(WebCore::ScrollableArea::horizontalScrollbarLayerDidChange):

This is for #4. This code computes the scrollbar’s value and current overhang 
amount.
(WebCore::ScrollableArea::computeScrollbarValueAndOverhang):
* platform/ScrollableArea.h:
(WebCore::ScrollableArea::layerForHorizontalScrollbar):
(WebCore::ScrollableArea::layerForVerticalScrollbar):
(WebCore::ScrollableArea::layerForScrolling):

This is for #1. We need to know if we have the ability to update scrollbars on a 
different thread. We can do that only on certain versions of the OS, only when 
threaded scrolling is enabled, and only when the current page is actually using 
the scrolling thread to scroll.
* platform/Scrollbar.cpp:
(WebCore::Scrollbar::supportsUpdateOnSecondaryThread):
* platform/Scrollbar.h:
* platform/ScrollbarThemeClient.h:

New ScrollbarPainter APIs.
* platform/mac/NSScrollerImpDetails.h:

This is for #5, letting the ScrollbarPainter API  know about the layers.
* platform/mac/ScrollAnimatorMac.h:
* platform/mac/ScrollAnimatorMac.mm:
(-[WebScrollbarPainterDelegate layer]):
(-[WebScrollbarPainterDelegate convertRectToLayer:]):
(-[WebScrollbarPainterDelegate shouldUseLayerPerPartForScrollerImp:]):

Before we kick off a scroll animation, set the current painting characteristics so 
they are up-to-date in case we are scrolling on the scrolling thread.
(-[WebScrollbarPainterDelegate setUpAlphaAnimation:scrollerPainter:part:WebCore::animateAlphaTo:duration:]):
(-[WebScrollbarPainterDelegate scrollerImp:animateKnobAlphaTo:duration:]):
(-[WebScrollbarPainterDelegate scrollerImp:animateTrackAlphaTo:duration:]):
(WebCore::ScrollAnimatorMac::didAddVerticalScrollbar):
(WebCore::ScrollAnimatorMac::didAddHorizontalScrollbar):
(WebCore::ScrollAnimatorMac::verticalScrollbarLayerDidChange):
(WebCore::ScrollAnimatorMac::horizontalScrollbarLayerDidChange):

Only paint the scrollbars through ScrollbarThemeMac if they are NOT being updated 
by the scrolling thread.
* platform/mac/ScrollbarThemeMac.h:
* platform/mac/ScrollbarThemeMac.mm:
(WebCore::ScrollbarThemeMac::setCurrentPaintCharacteristics):
(WebCore::scrollbarPainterPaint):
(WebCore::ScrollbarThemeMac::paint):

Back to #1.
* rendering/RenderLayer.cpp:
(WebCore::RenderLayer::updatesScrollLayerPositionOnMainThread):
* rendering/RenderLayer.h:
* rendering/RenderLayerCompositor.cpp:
(WebCore::RenderLayerCompositor::updateOverflowControlsLayers):
* rendering/RenderListBox.h:

Source/WebKit2: 

New pure virtual function.
* WebProcess/Plugins/PDF/PDFPlugin.h:



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@157253 268f45cc-cd09-0410-ab3c-d52691b4dbfc