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Looks like a custom glyph atlas cache based on top of FreeType [0].

These days I would use HarfBuzz/stb_truetype [1] for a lightweight thing. Use FreeType if you need something even fancier.

[0] https://git.blender.org/gitweb/gitweb.cgi/blender.git/blob/H... [1] like https://gist.github.com/rygorous/6f2779a451d2040371e3acb79e1...

Somewhere in the sources is the GHOST (General Handy Operating System Toolkit) folder which is where the UI stuff is implemented.

intern/ghost if I were to guess without looking at the actual sourcetree...

If you are into opengl guis, you can check ImGui, it is a joy to work with it. although it doesn't have any fancy font stuff afaik. Also there is UE4 slate, which has a funny way of doing stuff.