That window exits Split View. The other window switches to full-screen view. You can switch to the full-screen window with Mission Control, or use a Multi-Touch gesture such as swiping left or right with four fingers on your trackpad.

In Split View, you can use both apps side by side, without the distraction of other apps. Choose a window to work in by clicking anywhere in that window. Show the menu bar by moving the pointer to the top of the screen. Swap window positions by dragging a

Here's how to use Split View on a Mac. ... Just note that some apps — like Apple’s Photos, for example — have minimum widths, so you may not be able to adjust the bar much or even at all.