Bug report

So far, I've only seen this on macOS CI. test_never_update crashes and thus fails.

On #18235, I had to restart macOS on Azure 4 times to pass. In #18243 it failed on Travis and Azure, but only macOS.

cc @richardsheridan

Matplotlib version

• Operating system: macOS, mostly
• Matplotlib version: master-ish
• Matplotlib backend (print(matplotlib.get_backend())): TkAgg
• Python version: 3.7.8 / 3.8.5