I have several computers that have the same problem. At random a gray bar is displayed on top of the current windows explorer but only when it is maximized. IT can also be that on the bottom of the screen a bar with a part of the previous active program is displayed.
The only thing that helps is rebooting the computer.
I already tried to update all drivers where possible. I uninstalled all software they don't need. But still the problems persist.
It are all HP machines, 3 laptops and 1 desktop.
Below you can see some screenshots.