The original Mac OS supported a single app plus "desk accessories" which relied entirely on every application calling them at exactly the right points in their

The original Mac OS supported a single app plus "desk accessories" which relied entirely on every application calling them at exactly the right points in their own main event loop. It was a very limited form of "cooperative multi-tasking" which isn't really multi-tasking at all IMO. If any app or desk accessory got even the slightest part of the scheme wrong, which was easy to do because the "rules" were complex and unclear, it would take down the whole lot. No background threads, or really threads at all. Even contemporary DOS TSRs did better. Switcher came fairly shortly after, but (as the linked article makes clear) it was definitely a hack. Real multi-tasking didn't come to the Mac for a long time.