Using flick() on an atom with overlays/underlays will break overlay/underlay animations.
Numbered Steps to Reproduce Problem:
Add an overlay with a looping icon_state to an atom.
Have that atom flick() a different icon_state.
The overlay's animation loop will jump out of sequence.
The overlay/underlay should not be effected unless it has an icon_state matching what is sent through flick().
Whatever the overlay/underlay is currently looping through will flick() regardless of the icon_state.
Did the problem NOT occur in any earlier versions?
I'm not sure but, I remember at one point overlays/underlays could have their animation completely unaffected by flick(). A lot of people have even asked for a way to flick() overlays themselves.