> wanting to composite application together from smaller bits in a unified workspace that I the User control

This bit reminds me of TRON/BTRON, a Japanese OS where a core principle was that users interacted with documents made of fragments which were documents themselves.

"The file and the application were implementation details […] the user-visible primitive on a desktop computer shouldn't be a file or the application, but the typed document part."

https://www.xda-developers.com/japan-tried-build-operating-s...