[Webkit-unassigned] [Bug 176858] New: div with animated 'transform' should remain a containing block for positioned descendants, even if it's animating the value "transform:none"

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=176858

            Bug ID: 176858
           Summary: div with animated 'transform' should remain a
                    containing block for positioned descendants, even if
                    it's animating the value "transform:none"
           Product: WebKit
           Version: Safari 10
          Hardware: Unspecified
                OS: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Animations
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: dholbert at mozilla.com
                CC: dino at apple.com

I'm using Safari 10.1.2.

STEPS TO REPRODUCE:
(1) Visit https://jsfiddle.net/kkpznn6L/

EXPECTED RESULTS:
No red should be visible.

ACTUAL RESULTS:
Red is visible. (The fixed-pos lime div abandons its red containing block and uses the viewport as its containing block instead.)


Notes:
The animation ends at "transform:none", which normally (if there weren't any animations) would mean that the red div should stop being a containing block for positioned children, which would mean that Safari's behavior here is correct [1].

However, that spec text doesn't matter because there's another effect that supercedes it, from the CSS Animations spec:
"While an animation [...] has an animation-fill-mode of 'forwards' or 'both', the user agent must act as if the will-change property ([css-will-change-1]) on the element additionally includes all the properties animated by the animation."
https://drafts.csswg.org/css-animations/#animations

So the rendering should be the same as what it'd be if "will-change:transform" were present.  For comparison, here is a reference case with that style manually added:
https://jsfiddle.net/kkpznn6L/1/

Safari renders *that one* correctly (no red). So, that means Safari is mis-rendering the original testcase (https://jsfiddle.net/kkpznn6L/ ) -- it's not honoring this bit of CSS Animations spec text.

Firefox 55 and Edge 15 each produce the expected result, BTW.
Chrome 62 dev and Safari 10.1.2 produce the incorrect/unexpected result.
Chrome bug: https://bugs.chromium.org/p/chromium/issues/detail?id=764783


[1] "...any value other than 'none' for the transform also causes the element to ... [act] as a containing block for fixed positioned descendants."
https://drafts.csswg.org/css-transforms/#transform-rendering

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