UI programming - and GUI toolkit programming - is all about breadth. There’s a ton of things a UI needs to do, but most of those things are straightforward to implement, and there’s usually a plethora of lower-level libraries to compose in any given area. So, it does take a lot of work to implement a UI (toolkit) but not much of it is a stumper. And, many areas of UI toolkit functionality can be ignored or have low-quality implementation for a long time. For example, a hobbyist writing their own UI toolkit can ignore accessibility, but still end up with something usable by the hobbyist. Or, the initial implementation can use only absolute layout. Or, the initial implementation can use only a single fixed-width bitmap ASCII font.