Easy! But hard to tell if it worked - is it OK just to assume the preprocessor works because it works on other functions?

I'll answer that with a yes. The preprocessor itself is tested. We don't want to test every variant of possible calls to the plotting functions.

Sure, just wanted to make sure it was tested somewhere.

Failing tests are:

lib/matplotlib/tests/test_animation.py::test_draw_framelib/matplotlib/tests/test_arrow_patches.py::test_fancyarrow[pdf]

They're unrelated and passing here for me on OSX on Python 3.8...so do we ignore or re-run?

Those also failed on#18100. So I don't think it is these PRs, though its possible it is a previous PR that has worked its way into master...