I noticed that when I try to view a video or pdf file online in full screen, the browser ask me to confirm that I chose full screen.
At least I saw this on
Is there a security issue associated with this or it is simply a double check?
It's because going full screen without asking would confuse/annoy the user and could possibly be used to fake a windows desktop or password prompt to collect passwords. Also because switching to fullscreen can sometimes crash graphics cards. Pick a reason - they're all good.