"released" handler for windows or viewport, or deactivated handler for those items.

Firing an event after a window's dimension or position is changed by the user would be a useful feature.
I have an image resize function that's somewhat expensive so it would be good if there was a way to defer it until the window is released after the user resizes it rather than firing continuously. I noticed you can not add a deactivated handler to those items. Is there a way in the current API to do this? Otherwise I think that would be a useful feature.