Using the crop tool on an image currently disables the cursor keys’ ability to scroll through thumbnails. In order to reactivate the cursor keys one has to go back to a thumbnail and click on it. This is a pretty annoying waste of time and it interrupts the culling workflow. There’s no reason that this should happen since the cursor keys do not affect cropping.
The same thing happens after you've edited metadata, you then have to click elsewhere to be able use shortcuts again.