1133256 - Fixed positioned element inside an element with transform is treated as absolute positioned

Status: RESOLVED INVALID

(Core :: Layout, defect)

Description

[dror3go]

User Agent: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:35.0) Gecko/20100101 Firefox/35.0
Build ID: 20150125222008

Steps to reproduce:

Setting an element as position: fixed; inside an element with transform causes the browser to render it as if it was set to position: absolute;
A fixed positioned element should be positioned based on the viewport.

Comment 1

[David Baron :dbaron: (⌚️UTC-4, no longer working on Mozilla)]

This behavior is required by the spec; there are good reasons for it, although I don't have time to go into detail right now.  (Roughly, I think, it's not clear how it could be both affected by the transform and fixed-positioned in a reasonable way.)

http://dev.w3.org/csswg/css-transforms/#transform-property : "The object acts as a containing block for fixed positioned descendants."

I also expect that at this point there's enough Web content depending on this behavior that we probably couldn't change it if we wanted to, so it's probably not all that useful to discuss changing it on www-style.

Comment 2

[dror3go]

One can argue how many websites depend on this behavior. I don't think it's that many.
Also, not fixing due to such reasons is problematic. After all, we all expected Microsoft to fix their famous IE box model bug, right?

Having said that, I guess that if that's required by the spec then there's not much I can do :-)