User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:56.0) Gecko/20100101 Firefox/56.0 Build ID: 20170627100221 Actual results: See video. The upper Nightly is without stylo (expected), the lower one is with stylo enabled (bug). Notice: This bug occurs only from the second time on I click on "Download" (as you can see). (Webrender+webrendest are irrelevant because the behaviour is the same.)

Brian is investigating, may reassign to someone on the animations team.

(Emilio Cobos Álvarez [:emilio] from bug 1376788 comment 2) > The video player CSS is loaded with the same JS script mentioned in bug 1376092. Maybe this bug has the same root cause because a style tag was modified but not applied. But that would not explain (to me) that it is working (only) on the first click.

Nightly 56 x64 20170703100343 @ Debian Testing (Linux 4.9.0-3-amd64, Radeon RX480) about:support > Stylo: true (enabled by user) This bug is still present.

The disappeared dialog has a CSS Animation, its keyframes rule is; @keyframes rb-fx-zoomInSmall { from{ opacity:0; -webkit-transform:scale3d(.8,.8,.8); transform:scale3d(.8,.8,.8) } 50% { opacity:1 } } Creating the CSS animation on the second click, opacity of the target element seems to be computed 0, so the final keyframe (at 100%) value is 0. I have no idea where the 0 value comes from yet.

Will be fixed by bug 1360398. Honestly I don't quite understand why filling computed values in missing keyframes during generating keyframes pulls incorrect (stale?) style values, but bug 1360398 makes stylo match to gecko, so we should do it anyway.

I've confirmed that the patches for bug 1360398 fixes this bug.

(Hiroyuki Ikezoe (:hiro) from comment #6) > I've confirmed that the patches for bug 1360398 fixes this bug.

Verified fixed by bug 1360398. Nightly 56 x64 20170711100226 @ Debian Testing (Linux 4.9.0-3-amd64, Radeon RX480)