[cosmic_cheese]

Right, but my point is that users didn’t have to write extensions because developers had already written one for just about any niche use one could think of.

And it wasn’t just theming. Classic Mac OS extensions could do anything from add support for new hardware to overhaul the text rendering system entirely to giving dragged desktop icons gravity and inertia to adding a taskbar or a dock. The sky was the limit, and having a single common target to do any of those things (vs. being split between the kernel and a thousand layers/daemons/DEs/etc) meant that if it could be done, it probably had been.

[rbanffy]

You’d need to touch many different parts of the OS to write those extensions. The difference is that, on MacOS classic, there isn’t much of a boundary between user space and kernel space.

I’ve done a couple MITM toys with Windows 3.x and the trick is always exposing the same interface as the thing you want to replace, even if you only do something like inverting mouse movements on odd minutes, you just pass everything else down to the original module.