David Baron :dbaron: (⌚️UTC-4, no longer working on Mozilla) Reporter
	Description • 9 years ago

The CSS WG resolved on 2015-07-01 to change position:fixed so that it always creates a true stacking context, for Web compat reasons.  (This is needed since Safari and Chrome have worked that way for years.)

	David Baron :dbaron: (⌚️UTC-4, no longer working on Mozilla) Reporter
	Comment 1 • 9 years ago

... note that I believe this includes when it's not actually "fixed" because it's inside something with 'transform'

	Robert O'Callahan (:roc) (email my personal email if necessary) Assignee
	Comment 2 • 9 years ago

https://treeherder.mozilla.org/#/jobs?repo=try&revision=33cc2889181a

	Robert O'Callahan (:roc) (email my personal email if necessary) Assignee
	Comment 3 • 9 years ago

Bug 1179288. Make position:fixed induce a stacking context. r=heycam

	Robert O'Callahan (:roc) (email my personal email if necessary) Assignee
	Comment 4 • 9 years ago

Try looks good.

	Cameron McCormack (:heycam)
	Comment 5 • 9 years ago

(In reply to David Baron [:dbaron] ⏰UTC+2 (busy Aug. 8-Aug. 30) from comment #1)
> ... note that I believe this includes when it's not actually "fixed" because
> it's inside something with 'transform'

What is this rule?

	David Baron :dbaron: (⌚️UTC-4, no longer working on Mozilla) Reporter
	Comment 6 • 9 years ago

(In reply to Cameron McCormack (:heycam) from comment #5)
> (In reply to David Baron [:dbaron] ⏰UTC+2 (busy Aug. 8-Aug. 30) from comment
> #1)
> > ... note that I believe this includes when it's not actually "fixed" because
> > it's inside something with 'transform'
> 
> What is this rule?

A number of properties (transform, ) specify that non-default values cause the element to create a containing block for position:fixed elements.  See nsStyleDisplay::IsFixedPosContainingBlock.

	Cameron McCormack (:heycam)
	Comment 7 • 9 years ago

But does that affect whether non-position:fixed descendants need to create a stacking context?  (I was reading into your comment 1 that to fix this bug we need to check something other than the value of position, but I'm not sure we do.  Maybe I am misinterpreting?)

	Cameron McCormack (:heycam)
	Comment 8 • 9 years ago

Comment on attachment 8648609 [details]
MozReview Request: Bug 1179288. Make position:fixed induce a stacking context. r=heycam

https://reviewboard.mozilla.org/r/16255/#review14615

If there is another case that needs handling we can do it in another patch.

	Robert O'Callahan (:roc) (email my personal email if necessary) Assignee
	Comment 9 • 9 years ago

url:        https://hg.mozilla.org/integration/mozilla-inbound/rev/89a5b5c8ce17eaefdea8178171567ee309bbf447
changeset:  89a5b5c8ce17eaefdea8178171567ee309bbf447
user:       Robert O'Callahan <robert@ocallahan.org>
date:       Mon Aug 17 11:02:54 2015 +1200
description:
Bug 1179288. Make position:fixed induce a stacking context. r=heycam

	Ryan VanderMeulen [:RyanVM]
	Comment 10 • 9 years ago

https://hg.mozilla.org/mozilla-central/rev/89a5b5c8ce17

	Xidorn Quan [:xidorn] UTC+11
	Comment 11 • 9 years ago

roc, I guess we'd probably better backout this bug on 43 for regressions like bug 1201078, and reland it for 44 after the merge day. It seems top layer (bug 1126230) won't be able to catch 43 because of bug 1069192, which I don't think could land in this week.

	Robert O'Callahan (:roc) (email my personal email if necessary) Assignee
	Comment 12 • 9 years ago

OK.

	Robert O'Callahan (:roc) (email my personal email if necessary) Assignee
	Comment 13 • 9 years ago

Actually I think I'll back this out of Aurora after the uplift has occurred from Nightly to Aurora, rather than back it out on Nightly now and reland it after the uplift.

	Wes Kocher (:KWierso) (Not reading bugmail; email directly if needed)
	Comment 14 • 9 years ago

Backed out of Aurora in https://hg.mozilla.org/releases/mozilla-aurora/rev/2c8e89fa7719

Adjusting the status flags as I think they should be. Feel free to correct them if I thought wrong.

	Jean-Yves Perrier [:teoli]
	Comment 15 • 9 years ago

I've updated: https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Positioning/Understanding_z_index/The_stacking_context
https://developer.mozilla.org/en-US/docs/Web/CSS/position
and
https://developer.mozilla.org/en-US/Firefox/Releases/44#CSS

	Kohei Yoshino [:kohei]
	Comment 16 • 9 years ago

The site compatibility doc is up: https://www.fxsitecompat.com/en-US/docs/2015/css-position-fixed-now-always-creates-stacking-context/

My Firefox app was broken with this change ;-)