Lacqit
Lacqit is an Objective-C library that provides both UI and model classes. (The name of the library is an awkward amalgamation of "Lacquer Kit".)
Lacqit has dependencies on the Lacefx and LacqJS libraries, also available under the same MPLv2 license.
Lacefx is a C library, so some of the classes provided in Lacqit are Obj-C convenience wrappers for Lacefx objects and concepts. These are identified by the "LQLX" prefix (e.g. "LQLXSurface").
macOS status
The library is actively used on macOS. It builds for x86 and ARM64 (Apple Silicon).
Binary builds for the dependencies are provided in the Frameworks folder.
Windows support
At one point there was a Windows+Linux port of Lacqit that used GTK+ for its GUI implementation. Some of the class hierarchies in Lacqit are explained by this history: for example the view controllers had both AppKit and GTK+ implementations. This is why you'll see the "_cocoa" suffix on some classes.
There are also Win32-specific classes included, for example the LQLacefxView class has separate implementations for both Cocoa OpenGL and Windows Direct3D.
No project actively uses the Windows version of the library, so these builds are unsupported.