How to close split window / adjacent editor - Xcode 4

I've managed, by playing around wi开发者_运维知识库th the menus in Xcode 4, to open a window in an adjacent editor (Navigate, open in adjacent editor). Unfortunately, I'm having trouble figuring out how to close that window. How do you do that?

Cmd+Enter

or View -> Standard Editor -> Show Standard Editor

I fixed this by going to 'view > editor > standard' and this reset the view to the standard one window view. I have yet to figure out why the x in the upper right corner of the new window is grayed out. Not a perfect solution, but a solution.

There's a close button in the upper right corner of an assistant editor to close the editor.

The first assistant editor doesn't have buttons to add and close editors, which can be confusing.

I am using XCode 13.41. There is an "X" icon at the top left corner of the editor view. (You can hover on it to reveal what it does). Click on the X to close the editor.