few days ago my drivers where updated since that whenever I full screen youtube video (or another video in browser) my monitor is switched off immediately when button bar is hidden, if I leave my cursor on bottom bar (bar is not hidden) then the monitor is not switched off. I tried with Chrome and Edge - same result.
Apparently drivers problems.
I have this laptop:
you can see the specification on the right:
intel hd graphics 530: 26..20.100.7263
Nvidia geforce gtx 960M 188.8.131.5275
Thank you for your help!