When shift clicking items, items end up flashing because they are being moved by vanilla and then moved again by StackableItems.

It should be possible to check whether certain actions would work in vanilla and then let vanilla handle it instead of overriding anything.

So far, I've been able to let vanilla handle most actions when it can. I'm sure I haven't gotten them all yet and it will need some good testing to make sure everything is still working as it should.