• Vlang uses Sokol to draw UI
• Sokol uses DirectX/OpenGL/Metal
• Racket uses Cairo
• Check rxi/lite editor (made in Lua with Microui: https://rxi.github.io/index.html)
• If web stuff: https://surf.suckless.org/, Webview by zserge
• Red/Rebol?
• Game engines (Godot is apparently nice, update 07/2021: I tried it and it's actually nice!)
• Libui
• https://gioui.org

Related to

• BEdit UI
• Webview by zserge
• Skija: Graphics Library for the JVM by JetBrains (Nikita)
• 7GUIs