I don't have a scientific basis for this, only observations of my own cats.
It could be that she knows that things are always moving around on the screen, and in their absence, she is trying to coax them out of hiding, or figure out how to bring them back.
Especially if she is bored. When my cats are bored they'll start falling back on other things that have been interesting in the past; windows, the projector screen, iPads, the bathtub faucet, the printer, that random place they chased a fly around in the past, the spot on the wall I always shine the laser pointer, etc. And cats will often paw at things to try and "activate" them or whatever, since they can't think of much else to try.
I interpret that kind of pawing as a general "I want the thing that was interesting here back again" or "I know there is something about this spot, what is it?"
Distracting them with play has always worked for me, and keeping them busy seems to decrease the behavior overall.