Scroll the following page with either paint flashing and/or omtc+layer border: http://www.just-eat.ca/restaurants-dim-sum-king/menu-5306 Notice the div absolute positioned CategoryPanel & divBasketUpdate (Top left & Top right) don't get a layer.

Abspos stuff does not necesasarily form a stacking context, so isn't painted atomically, no?

I just checked in chrome and they don't build a layer for it either. Could be an invalid bug. I'm not familiar with stacking context. Is it hard/impossible to layerize this div?

This testcase should make it clear why you can't layerize abspos things in general if they have auto z-index...

Right I agree that z-index is a major problem but I don't see it in the page. Its difficult to find a reason why this isn't being layerize to see if this is or isn't the case.

I _think_ that an abs-pos thing might be ok to layerize as long as no descendants of it have non-auto z-index... But it's worth double-checking this carefully against http://www.w3.org/TR/CSS21/zindex.html

We should actually be able to layerize here. We don't depend on content being a stacking context to be able to layerize it. I'm not sure why this isn't being layerized.

The reason we're invalidating all the time is that they're doing a poor-man's position:fixed by constantly adjusting 'top' to keep the "Categories" element at the top of the page while you scroll :-(. We should layerize for abs-pos 'left'/'top' animation.

We'd need a fix for bug 745485 to fix the testcase in comment #0.

Ah, we already have a bug for layerizing left/top.

Bug 876321 fixed this, as far as I can tell.