1791884 - Make sure we respect animation compositor order for scroll-linked animations with delay on the compositor

Status: NEW

(Core :: CSS Transitions and Animations, enhancement)

Description

[Boris Chiou [:boris]]

This is a complicated issue which noticed in https://phabricator.services.mozilla.com/D155108#inline-868474. And the possible case was mentioned here: https://phabricator.services.mozilla.com/D155108#inline-868474.

We can easily meet the both conditions with a long positive delay document timeline animation which replaced an old animation on the same element and with a scroll linked animation on the same element with positive delay. In such cases we set ScrollToDelayPhase, but we can't tell whether the document timeline animation should be prior or the scroll timeline animation should be prior in this function because we've lost the composite order information unfortunately.

We need to revisit this anyway.