This is great -

It’s almost as impressive as the “GOS” written for Atari 8-bit running on 6502 at 1.79 MHz. Visually it’s almost identical but has a nicer font. The specs are somewhat similar with 16 task slots, calculator app and task manager, serial mouse support (ST mouse) and Atari’s hardware sprites helping with some of the graphics. The XOR window frame is the same idea. Amazingly, Atari GOS runs in 128K of RAM! One has to admire the work and inventiveness in these projects. See description and videos here:

https://atari8.co.uk/gui/