"apple has disabled your computer" = scam

Browsing to FaceBook on iMac, I got a "facebook has temporarily suspended your account" popup, with an address in the address bar like "hammerhead-app-6cpo2.ondigitalocean.app". Something it put my browser in fullscreen mode and flashed me a big scary fullscreen warning that "Apple Has Disabled Your Computer", and that I should call Apple Support Toll Free +1-888-808-4228. The number shows up as a known scam, I think some sort of pay to dial thing.

I have never seen my browser (Safari) taken over like that. Going into full screen mode and my external monitor going dark is pretty alarming.

Not sure what my question is. Anyone else had this happen? How can a website tell my browser to go into fullscreen without warning me or getting permission?

Or maybe this is my life's purpose: serving as a warning to others.