I get that the point of handmade is to build from scratch, but often a great way to learn is to dig into other peoples code.
The point of Handmade (at least as per my interpretation) isn't to build everything yourself from scratch. It is to have an understanding of what is occurring under the hood when your program runs.
If everyone wrote everything from scratch, we wouldn't get anything done. We as programmers already spend too much time independently solving the same problems over and over again. < /rant >
I've been using Omar Cornut's
ImGui to make UI for an image editor I'm working on, and I've found the source code to be very readable, and I've also picked up a number of C/C++ tricks from it. I highly recommend you take a look if you haven't already.
Sean's stb libraries are hugely awesome to work with, but they have been optimized for performance and extensibility (e.g. custom allocators and various macro-controlled options), and can be a bit formidable for newcomers to dig into.