CSS rotation transform can cause elements with certain styles to vanish during rotation.

While an element is rotating, any element on the page that has no height, no width, is absolutely positioned, and has overflow set to hidden will vanish for the duration of the animation.
I found this because a crucial element on a page had all those, yet still had a border, and then disappeared.

[Mark Rowe (bdash)]

<rdar://problem/8207271>

[Simon Fraser (smfr)]

Created attachment 61993 [details]
Testcase

[Simon Fraser (smfr)]

The disappearing element has zero height and width but a large border. clipRectToApply ends up as a 0x0 rect in RenderLayerBacking::paintIntoLayer().

[Simon Fraser (smfr)]

Hmm, RenderBlock::paint() is short-circuiting the paint here:

    // Check if we need to do anything at all.
    // FIXME: Could eliminate the isRoot() check if we fix background painting so that the RenderView
    // paints the root's background.
    if (!isRoot()) {
        IntRect overflowBox = visibleOverflowRect();
        overflowBox.inflate(maximalOutlineSize(paintInfo.phase));
        overflowBox.move(tx, ty);
        if (!overflowBox.intersects(paintInfo.rect))
            return;
    }

paintInfo.rect is the empty rect from clipRectToApply

[Simon Fraser (smfr)]

Ah, this is caused by the 'inflate the paintBox for the body' logic in RenderLayerBacking::paintIntoLayer().

[Simon Fraser (smfr)]

Created attachment 62144 [details]
Patch

[Simon Fraser (smfr)]

http://trac.webkit.org/changeset/63820