We keep a set of properties which are running on the
`CascadeLevel::Animations`, to make sure we compose the correct properties
for the rule on Animations/Transitions cascade level.
In other words, without this patch, we may accidentally compose an
animation rule for `CascadeLevel::Animations` (from the KeyframeEffect of
the existing transition), even if we don't have any running animations,
if this transition property is a custom property.
The resolution is to use `AnimatedPropertyIDSet`, for
`mPropertiesForAnimationsLevel`, so we can skip the custom properties which
shouldn't belong to Animations cascade level when composing them.
Note that we don't change the logic, and we are still using
`nsCSSPropertyIDSet` for `EffectSet::mPropertiesWithImportantRules`
because compositor animations don't allow custom properties now.
(This could be a future work I guess, if we are using custom property
values for opacity, translate, rotate, scale, etc.)
Also, I noticed the subtest, "No transition when removing @property rule",
is similar to the testcase in this bug, and so it got passed as well.
Differential Revision: https://phabricator.services.mozilla.com/D210589
91be0d7215