rxi/microui — reverse-engineered prompt

Build me a tiny portable UI library in ANSI C that works in an immediate mode style and can plug into any renderer that knows how to draw rectangles and text. I want it to avoid heap allocation and run from a fixed memory buffer. Include the basic widgets from the screenshot and example, windows, scrollable panels, buttons, sliders, textboxes, labels, checkboxes, wrapped text, and simple popup support. It should have a simple layout system and be easy for someone to add custom controls later.

Please also include a small demo app that shows the widgets working and exposes events like button presses. The library should only manage input, state, and drawing commands, not do the actual drawing itself. Keep it small and lightweight, with clear usage notes so I can drop it into another project. If you need details, check the existing docs and demo and current references online.