It sounds like "full screen." There are two kinds: * Content full screen, like clicking the full screen button on a video. You can escape from this kind of full screen by pressing the Esc key, or sliding the mouse pointer to the top of the screen and watching for a button to slide down. * Browser full screen, which on Mac is turn on and off by Comand+Shift+f. Or you can slide the mouse pointer to the top of the screen and watch for the whole toolbar area to slide down, then click the menu button and use the last icon on the row that starts with "Zoom". Was that it?