Stand up. It's impossible for her to keep that pose if you stand up for a moment. You can either say you are just going for a coffee and does she want one or just stand up and stretch or stand up and lean across your desk to pick up an item further away (keep a pencil pot out of arm's length for this purpose).

By doing this repeatedly you will eventually train her brain into the routine. She touches you on the shoulder to get your attention, then her hand comes off your shoulder because you stand/because that's what happens.