I would like to detect when an external window is updated|redrawn. In other words, I want to catch some kind of an event whenever a specified window (not of my app) changes its content (e.g. by updating nested image, displaying new text message, layout change etc.). This question applies both to X11/GTK and win32 based systems. Do you know any way to achieve this?
With X11 you need to use Damage extension - DamageCreate / DamageSubtract requests and DamageNotify event. Not sure about gtk api ( Ideally there should be wrapper around X11/Damage and win32 but not sure if it exist ) - try to look at damage-event