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 Travisand 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