You actually mis-remember. One of the things that was a perpetual telephone-game distortion during the Operating System Wars was people talking about a single input queue.

Presentation Manager did not have a single input queue. Every PM application had its own input queue, right from when PM began in OS/2 1.1, created by a function named WinCreateMsgQueue() no less. There were very clearly more than 1 queue. What PM had was synchronous input, as opposed to asynchronous in Win32 on Windows NT.

Interestingly, in later 32-bit OS/2 IBM added some desynchronization where input would be continued asynchronously if an application stalled.

Here's Daniel McNulty explaining the difference in 1996:

* https://groups.google.com/g/comp.os.os2.beta/c/eTlmIYgm2WI/m...

And here's me kicking off an entire thread about it the same year:

* https://groups.google.com/g/comp.os.os2.programmer.misc/c/Lh...

Because of that, I got good at creating multi-threaded GUI apps. Stardock were champs at this - they had a newsgroup reader/downloader named PMINews that took full advantage of multithreading.

The rule of thumb I had heard and followed was that if something could take longer than 500ms you should get off the UI thread and do it in a separate thread. You'd disable any UI controls until it was done.