Classic MacOS did the same thing. Basically, all GUI processes call GetNextEvent() at the head of the event loop. In 1985 Andy Hertzfeld wrote Switcher which

Classic MacOS did the same thing. Basically, all GUI processes call GetNextEvent() at the head of the event loop. In 1985 Andy Hertzfeld wrote Switcher which used the GetNextEvent() entry point to pause an app and allow another app to run until the next call. Hertzfeld got the idea from the DOS utility Memory Shift. https://www.folklore.org/StoryView.py?project=Macintosh&story=Switcher.txt https://www.folklore.org/StoryView.py?project=Macintosh&stor... With well behaved apps it worked remarkably well. Apple brought it into the OS as MultiFinder.