MKWidgets is a widget based UI framework that predates vanilla Minecraft's own widget overhaul. It integrates seamlessly with existing Minecraft widgets while providing a much more advanced composable widget system using constraint-based layouting. With the use of MKWidgets we have managed to migrate our UI from 1.12 all the way through to 1.21 without having to change the UI code itself.
The architecture differences were inspired by my many years on the Kivy UI project and usage of Apple's constraint layout system.
