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