I can confirm that this:

• logged in (but locked) with screen plugged in -> True
• logged in (but locked) with the hdmi unplugged -> True
• logged out -> False

Running the tests on this branch locally without a user logged in to the desktop (so launched via ssh) I get:

ERROR lib/matplotlib/tests/test_backend_macosx.py::test_cached_renderer - ImportError: Cannot load backend 'macosx' which requires the 'macosx' interactive framework, as 'headless' is currently running
ERROR lib/matplotlib/tests/test_backend_macosx.py::test_savefig_rcparam - ImportError: Cannot load backend 'macosx' which requires the 'macosx' interactive framework, as 'headless' is currently running

This suggests that what is going on the CI is slightly different than not having a display?

On Actions, the display does appear to exist, according to the results from #27723:

mainScreen: Built-in Display (1920.000000x1080.000000)+0.000000+0.000000 @8
deepestScreen: Built-in Display (1920.000000x1080.000000)+0.000000+0.000000 @8
Available screens: 1
    0: Built-in Display (1920.000000x1080.000000)+0.000000+0.000000 @8

For your failures, I think that's because the macOS tests don't have a skip, so I hadn't added one (only modified existing ones.)

I tried re-running the failed mac3.11 jobs.

Looking at https://stackoverflow.com/questions/71954472/tkinter-crashes-on-ipython-but-not-in-python-on-a-m1-mac I think this might be because NSApplicationLoad messes with Tkinter somehow. I thought they were shared instances, but there must be something Tkinter is doing that confuses it if we create one ourselves.

Ah, here's a full explanation:

• Both pyglet and Tk use AppKit to implement their GUI
• AppKit uses an NSApplication class, and in particular a singleton instance of that class, to represent the application, which will get instantiated during application startup
• The Tk library uses a sublclass of NSApplication with additional functionality (TkApplication).
• When pyglet is started first the NSApplication singleton is an instance of NSApplication and not of TkApplication, but Tk's implementation assumes that the singleton is an instance of TkApplicationo.

but substitute the new code written here for pyglet above.

AFAICT from Googling though, we need the NSApplication instance to be sure that the NSScreen attributes are filled, though I've not tested it out myself.

Can you check if it works without the NSApplicationLoad?