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The classic Mac OS (Toolbox) menu routine took over exclusive use of the machine when it was tracking the mouse in the menu - all multitasking stopped running.

So an extension could draw whatever fancy effect it wanted when the menu was down without worrying about a background application drawing over it (drawing over the transparency) as long you made sure to restore what was beneath when the menu was let go.

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There were extensions that got around this, though. iTunes for the classic Mac OS (and I'm pretty sure SoundJam before it) could continue to play music with a menu open, for example.

Yeah you could do things like set timer interrupts, and starting in somewhere like MacOS 8.6 there was an actual multitasking (and multi-CPU) nanokernel running beneath everything that allowed you to schedule tasks in a more modern way.

But those tended to have some pretty gnarly limitations (like I think in interrupts you can't allocate memory) so AFAIK they were only used for stuff like real-time audio, I dunno if anyone ever used those to do screen drawing, so in practice I can't think of anything that would interfere with menu drawing.

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