Hi Max,
Certain elements are the developers' responsibility to position responsively. Basically, this means positioning by percentage of width/height rather than absolute pixel values. I'd recommend taking a look at the Best Practices section of the documentation for an overview and examples.
While stretching the GUI layer to the window will work around the need for responsive design, it's not a true solution to the problem. However, you can disable GUI scaling in the xtend_config script if you'd like to manage it yourself.
Thanks for using Xtend!