Yeah very weird Qt bug. I have to think a workaround for that. It just doesn't refresh but it still working. You can click the X button and you'll see that the window close. You can even blindly move the selection rectangle and press in the ok button and you'll see it crops the image.

But it doesn't have anything to do with Yappari, sadly. Actually any Maemo notification causes it, if you receive an e-mail, SMS, or anything that causes a notification you'll see the bug.

So I don't know how to fix it or a workaround for it. In the meantime you can press the X (or where the X is supposed to be) to close the window and then open it again.