Bug 115141 – Rendering on http://www.milwaukeepolicenews.com is broken

• Component: Layout and Rendering
• Status: NEW

[Alexandru Chiculita]

Open http://www.milwaukeepolicenews.com and start scrolling. Looks like the tile rendering is broken for some of the elements (enable the layer borders to see that).

[Alexandru Chiculita]

Created attachment 206380 [details]
Test case

Simplified test case that reproduces the issue.

The issue is triggered by the fact that now the fixed elements will always force a composited layer. Because of that the parent that sets the clipping also gets a composited layer. Looks like composited fixed elements inside other composited clipping layers was broken in Safari 6 too, but the fixed element was not converted to composited mode.

The bug also reproduces in Chromium. Firefox 22 and IE11 do not reproduce the issue and do not clip the fixed element.

[Simon Fraser (smfr)]

Yeah, fixed inside of clipping is broken.

[Alexandru Chiculita]

(In reply to comment #2)
> Yeah, fixed inside of clipping is broken.

We could detect this case and apply the clipping on the children layers instead of the parent. It wouldn't work for transformed parents, but I think that's ok as that case wouldn't work in 2D mode either.

Do you want me to work on that or it's not worth the effort to support that edge-case?

[Simon Fraser (smfr)]

I think this is worth fixing.

The tricky part would be an animation on an element between the clipping element and the pos:fixed; we'd have to fall back to software animation in that case so that all the clipping layers can be updated for each frame.

But maybe this is the "transformed parents" case you mention. Also, don't forget that transforms act as containing block for fixed position.