BL-482: Scrollbar in image dialog is sticking

This is probably the one we want to use.
This is a busy-waiting loop that will keep one core busy indefinitely, wasting power. It might even make a single-core computer (if there still is such a thing) freeze. I think we should put a Sleep into this loop...even 50ms would make sure the dialog thread gets a chance and the main thread core isn't working so hard, yet without slowing things much.