1519838 - Background-position fails to animate under special circumstances

Status: RESOLVED FIXED

(Core :: CSS Transitions and Animations, defect)

Description

[Christian Sonne [:cers]]

Attachment: Simplified test case

First discovered here: https://twitter.com/anatudor/status/1083816059751284736

Under very specific circumstances, background-position fails to animate.
Curiously, it animates properly if you click (and hold) somewhere inside the document and move your mouse continuously.

STR:

1. Load attached test case

What should happen:

You should see two squares with animating backgrounds, the bottom one slightly masked.

What happens:

Unmasked square animates correctly, masked one doesn't animate at all.

I've tracked the regression via mozregression to https://hg.mozilla.org/integration/mozilla-inbound/rev/d2218beee052 landed in bug 1494092.

Comment 1

[Jonathan Watt [:jwatt]]

Thanks for the bug report, and especially the reduced testcase!

Comment 2

[Emilio Cobos Álvarez (:emilio)]

Hmm, this seems to work with WebRender... that's weird.

Comment 3

[Christian Sonne [:cers]]

(In reply to Emilio Cobos Álvarez (:emilio) from comment #2)

Hmm, this seems to work with WebRender... that's weird.

I don't know if it's related, but the original example from twitter fails differently in WebRender.
The background animation seems to work as intended, but the rotation is wonky.