882390 - Position absolute content doesn't get layer causing excessive invalidation

Status: RESOLVED WORKSFORME

(Core :: Layout, defect)

Description

[Benoit Girard (:BenWa)]

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.

Comment 1

[Boris Zbarsky [:bzbarsky, bz on IRC]]

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

Comment 2

[Benoit Girard (:BenWa)]

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?

Comment 3

[Boris Zbarsky [:bzbarsky, bz on IRC]]

Attachment: Testcase that shows that you can't layerize abspos things in general

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

Comment 4

[Benoit Girard (:BenWa)]

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.

Comment 5

[Boris Zbarsky [:bzbarsky, bz on IRC]]

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

Comment 6

[Robert O'Callahan (:roc) (email my personal email if necessary)]

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.

Comment 7

[Robert O'Callahan (:roc) (email my personal email if necessary)]

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.

Comment 8

[Robert O'Callahan (:roc) (email my personal email if necessary)]

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

Comment 9

[Robert O'Callahan (:roc) (email my personal email if necessary)]

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

Comment 10

[Markus Stange [:mstange]]

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