While investigating https://bugs.webkit.org/show_bug.cgi?id=86025 Simon and I discovered another strange bug with vertical text and compositing. Open the attached test case in Safari. Scroll to the left to reveal the rest of the green box, and notice that instead of the expected white background, you end up seeing the green box over a shadow or dark linen or something.
Created attachment 141042 [details] Test case
<rdar://problem/11419933>
Created attachment 142624 [details] Account for flipped writing modes in RenderLayer::calculateLayerBounds()
Fixed in r117537. <http://trac.webkit.org/r117537>