My mill was lacking something very important: the drives would remain
powered, and drifting due to test/offset, when EMC2 was not commanding
any motion.

Just to clarify: if EMC is holding position, then the drives are held
in position by EMC. But if EMC is not holding position, for example
because it stopped due to following error, then the drives would start
drifting.

It happened a couple of times already that I would forget about EMC
left without controlling position, and the mill would drift next day
to some unacceptable position and trip the limit.

I finally added an extra instruction to "inhibit" the drives when the
axes were not position controlled. Now, if EMC is stopped due to
following error, or I turned the motion off, the drives are inhibited
and would not do anything.

i