If progress is not NULL, the animation progress will be returned
there, 0.0 means the state is closest to being unset, while 1.0 means
it’s closest to being set. This means transition animation will
run from 0 to 1 when state is being set and from 1 to 0 when
it’s being unset.
Returns TRUE if there is a transition animation running for the current region (see Style.contextPushAnimatableRegion).
If progress is not NULL, the animation progress will be returned there, 0.0 means the state is closest to being unset, while 1.0 means it’s closest to being set. This means transition animation will run from 0 to 1 when state is being set and from 1 to 0 when it’s being unset.