'Stage watcher' is causing a problem for 'at' block

See how - in this project - a 'stage watcher' (which is needed on stage to allow users to select another 'input image' anytime) is causing problems for the 'at' block because the latter seems to be somehow sensing the former or, at last, the 'at' block can not do its proper job (i.e. sensing the pentrails/sprites/etc).