What makes the row of tabs, the menu bar, the address bar etc. disappear northward?

F11

The F11 key invokes full-screen mode in Windows (and freaks people out)

Thank you @jaytkay

@jaytkay It’s a favorite trick of mine to screw with the unaware :D

@jaytkay which key/s are involved in creating the problem (the disappearance) in the first place?

@flo Same key, F11 toggles between normal and full-screen

@jaytkay Okay. Thanks again.
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@flo It really isn’t a problem unless you do it unexpectedly. I used it fairly often in my browser and all the time in image/video-related programs. When I had my old netbook that had a 1024×600 screen, many web pages and some applications were unusable without hiding the menu bars and all.
The only time it’s really a problem is when you forget/don’t know what key toggles it, but most programs that support full-screen viewing use F11 to toggle it.

“It’s a favorite trick of mine to screw with the unaware :D”