Clay (short for C Layout) is a high performance 2D UI layout library.
See the website at
https://nicbarker.com/clay
for more info, or check out the github repository at
https://github.com/nicbarker/clay
A more in depth discussion of immediate mode UIs:
https://youtu.be/Z1qyvQsjK5Y
00:00 - Explanation of UI Layout Libraries
05:56 - Computing vs Rendering Layout
08:30 - Immediate Mode vs Retained Mode
16:05 - Code Examples & Quick Tutorial