This one is hard to explain. Hopefully, there are others with this issue. I use a Wacom Intuos 4 pen tablet for re-touching. I use it in wireless mode (with dongle). Every once in awhile Photoshop will lose focus. For instance, I'm using the pan/scroll function to move the image around or the brush tool. In the middle of this action I can no longer access the image on the screen. I can access menus and operate everything else but I can no longer do anything to the image. I can't move it, modify or or anything else. The only solution is to close the image and re-open it. It's difficult to reproduce because it's sporadic.
During the duration of this glitch, the pen also works fine outside of PS.
I've been experiencing this for a couple of years now but finally decided to bring it up here.
There was an updated driver posted about a month ago. I've installed it but I didn't see anything in the release notes about my issue. Hopefully, it helps and it just wasn't something addressed in the release notes.