Garbage at the top of http://www.technologyreview.com after scrolling

[Simon Fraser (smfr)]

http://www.technologyreview.com shows garbage at the top on Mac if you scroll down then back up. Regression from r146531

[Simon Fraser (smfr)]

<rdar://problem/13684286>

[Simon Fraser (smfr)]

The element showing the garbage pixels has visibility:hidden, which is not considered by RenderLayer::backgroundIsKnownToBeOpaqueInRect().

Another change I notice that affects this is https://bugs.webkit.org/show_bug.cgi?id=108118. Before that revision, we'd compute that the fixed position element is a simple container layer (yellow border). After that change, it became something that draws content.

I haven't been able to make a useful reduction. Most of the time, visibility:hidden just causes us to not parent the compositing layer.

[Simon Fraser (smfr)]

Created attachment 199573 [details]
Patch

[Tim Horton]

Comment on attachment 199573 [details]
Patch

View in context: https://bugs.webkit.org/attachment.cgi?id=199573&action=review

> LayoutTests/compositing/contents-opaque/visibility-hidden.html:49
> +    	<nav> 
> +			<ul>
> +				<li>Some text here</li>
> +			</ul>
> +    	</nav> 

TABS

[Simon Fraser (smfr)]

https://trac.webkit.org/r149084