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One of the tests (test_ttk_guionly.test_variable_change) on the Ubuntu CI is intermittently hanging on this code:
 https://github.com/python/cpython/blob/e42b705188271da108de42b55d9344642170aa2b/Lib/tkinter/test/test_ttk/test_extensions.py#L147
 def test_variable_change(self):
 x = ttk.LabeledScale(self.root)
 x.pack()
 x.wait_visibility()
 ^^^^^^^^^^^^^^^^^^^
The failure at https://github.com/python/cpython/runs/697113653 gave the logs:
 0:28:29 load avg: 0.00 [425/425/1] test_ttk_guionly crashed (Exit code 1)
 Timeout (0:20:00)!
 Thread 0x00007fa268d0c080 (most recent call first):
 File "/home/runner/work/cpython/cpython/Lib/tkinter/__init__.py", line 697 in wait_visibility
 File "/home/runner/work/cpython/cpython/Lib/tkinter/test/test_ttk/test_extensions.py", line 147 in test_variable_change
 ...
The same did not happen when I re-ran the CI checks. I thought I remembered a similar issue in the past, but I couldn't find an bpo issue about it.

By 'intermittantly', do you mean the one time on PR-20236 (and then not on your retest) or multiple times on various PRs and buildbots?
Serhiy, could the issue be that tk is waiting for an event that has already happened?

See also bpo-30756.

I listed this failure on the original test_ttk_guionly failure issue.