Off the top of my head, do a highlight pulse on the two snapped edges, and over the app window raise an ephemeral notification "New window snapped, click here o

Off the top of my head, do a highlight pulse on the two snapped edges, and over the app window raise an ephemeral notification "New window snapped, click here or press [hotkey] to undo". This is just the first thing I have thought of. There are probably better options that require a little more thought.