What is the visibility of open pop-up windows in userjs?

Question

JavaScript

I ran into a problem in Chrome. I don’t know how things are in other browsers, because I haven’t tested it, and Chrome is what I need.
We open a window:
var mywindow = window.open( ... );
further with mywindow I do everything that is necessary for me. When the script is connected in the usual way or, everything is fine. But it is worth making userjs out of it, then mywindow becomes undefined in any case, although the window opens.