Okay, wow, that’s in-depth. Glad you seem to like it!
I don’t have the mobile exports so I can’t test it (currently), are there any other changes that need to be made for it to work smoothly besides the window mouse functions and the on-screen keyboard? It may be worth me making a configuration specifically for mobile. Performance-wise it might also help to optionally use the built-in GameMaker text renderer instead of Scribble - Scribble is great on desktop, but the text shader is a bit beefy so it may not be worth it on mobile devices, or laptops with integrated GPUs - as you seem to be seeing.
Drop-down elements might be a good idea (I assume you mean something like these), I might add one next time I get around to implementing new features. Functionally they’d be pretty similar to single-selection lists, except that they’d be collapsible when not active. (The original UI code I wrote that this is based on actually had something like that, but I didn’t think it worked very well so I didn’t include it in here - probably worth redoing at some point.)
I’ve also been (slowly) trying to remove the amount of hard-coding, but as I see you’ve noticed there’s still quite a lot of it left. Adding anchor points and other support for scaling is also something I’ve thought about - I haven’t needed it myself so it’s kind of taken a back seat, but a few people have expressed interest in that sort of thing so I should probably work something out one of these days.