Well like e.g. when I had to write UI elements into first iterations of Star Citizen, those were all in a 2D vector/sprite layer over the 3D world that had to be completely custom coded. Even the shape of the UI was pretty much completely open to interpretation. I have mad respect for people who write their own UI elements and "widgets" into games, given the restrictions that are often involved. Some do it better than others, but therein lies the art of observing and stealing from the best.