Bug 90033 – Position fixed elements are incorrectly clipped in composited layers

• Component: Layout and Rendering
• Status: RESOLVED CONFIGURATION CHANGED

[Elliott Sprehn]

Created attachment 149651 [details]
Reduction

A position fixed element inside a composited layer will be incorrectly clipped to the containing positioned block. In the attached test case you should see two red boxes but Webkit only draws one because the other is clipped away.

Note: This doesn't appear to be a bug with stacking contexts. The page renders the same with or without a stacking context being created.

[Elliott Sprehn]

enne: Any idea what's up here?

[Adrienne Walker]

(In reply to comment #1)
> enne: Any idea what's up here?

This is on my radar, but I haven't had a chance to dig into this yet.

[Adrienne Walker]

(In reply to comment #2)
> (In reply to comment #1)
> > enne: Any idea what's up here?
> 
> This is on my radar, but I haven't had a chance to dig into this yet.

Now that I have a chance look into this, it seems that the second fixed position div in the test case appears to be clipped by its parent's overflow regardless of whether the page triggers accelerated compositing or not.  If it's not compositing-related, it's probably a bit outside what I know in WebKit.

[Ahmad Saleem]

Attached testcase appear same in Safari 16.4.1, Chrome Canary 115 and Firefox Nightly 114 and show 'red' end to end container / rect on top of page (window wide).

@Simon - if I am testing incorrectly, can you guide or else can we close, if all are matching?

[Simon Fraser (smfr)]

The transform now acts as the continuing block for fixed, so the second box is clipped to zero height by its container.