[Webkit-unassigned] [Bug 78872] [chromium] ScrollbarLayerChromium/CCScrollbarLayerImpl for CC-side scrollbar painting

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=78872





--- Comment #24 from Tien-Ren Chen <trchen at chromium.org>  2012-03-01 20:04:48 PST ---
(In reply to comment #23)
> (In reply to comment #22)
> > (In reply to comment #20)
> > > (In reply to comment #19)
> > > > > Source/WebCore/platform/graphics/chromium/ScrollbarLayerChromium.h:48
> > > > > +    RefPtr<Scrollbar> m_scrollbar;
> > > > 
> > > > this seems very wrong. the layer shouldn't have any ownership concern with the scrollbar.
> > > 
> > > the alternative would be copying everything when creating ScrollbarLayerChromium.
> > 
> > I find this won't work. m_visibleSize and m_totalSize is something very dynamic, we need to update their value on every commits.
> > I think we might be able to get these information from the layer data, any suggestion?
> 
> These are asking about the visible / total size of the scrollable area, not the scrollbar itself.  I think you should get those from the CCLayerImpl that's actually scrolling by giving the CCScrollbar (or CCScrollbarLayerImpl) a weak pointer to the CCLayerImpl that scrolls.  For example, totalSize() is some combination of the CCLayerImpl's maxScrollPosition, bounds, etc.
> 
> Keep in mind that these values can change on the thread without us going through a commit - for example, if the page scale changes due to a pinch gesture that changes the max scroll position and thus the totalSize() value. I think the only way to stay sane with these updates is to always query the value from the scrolling layer and not try to proactively sync them to other objects.

Ok, I managed to get all the values. (for totalSize again we use m_scrollLayer->children()[0]->contentBounds() hack)

However I observed that the scroll maximum is not 100% identical to the main-side. The impl-side scroll maximum is slight smaller than the main-side on http://www.nytimes.com/ This can be easily reproduced using arrow-key scroll (slow) and mouse-wheel scroll (fast).

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