On a more related note, I find it interesting that the GUI doesn’t appear to be a toolkit like QT, but lower level, like a GUI designed for in-game use.
This is the not-uncommon case where the irregular form is slowly losing ground to the regular form. It's possible that in a century, 'extensible' will be considered archaic.
[0] https://books.google.com/ngrams/graph?content=extendable%2C+...
I am considering using something like stretch[0] to handle layout concerns eventually.
I can't speak for this project, but what you're describing is an Immediate Mode GUI [1]. imgui is one that comes up a lot.