The Windows CI fails due to timeout.

[HideakiImamura]

Expected behavior

I have been observing an issue for the past few days where the Windows CI randomly fails due to timeout, regardless of whether it's a PR or on schedule. It looks something like the following CI:　https://github.com/optuna/optuna/actions/runs/6777581250/job/18421473256#step:8:15

All of the CIs appear to fail on the first test case of the visualization features using matplotlib. I have no windows machine to reproduce the error on my local environment.

Environment

• Optuna version: the current master https://github.com/optuna/optuna/commit/523dcc276a64e0d22ff8c92c17b88509b94bea88
• Python version: 3.9
• OS: Windows Server 2022
• (Optional) Other libraries and their versions:

Error messages, stack traces, or logs

Here is the CI's output.

Run pytest -m "not integration"
============================= test session starts =============================
platform win32 -- Python 3.9.13, pytest-7.4.3, pluggy-1.3.0
rootdir: D:\a\optuna\optuna
configfile: pyproject.toml
plugins: jaxtyping-0.2.23, typeguard-2.13.3
collected 4850 items / 839 deselected / 4011 selected

tests\test_callbacks.py .                                                [  0%]
tests\test_cli.py ...................................................... [  1%]
...
tests\trial_tests\test_trial.py ........................................ [ 80%]
........................................................................ [ 81%]
..............................                                           [ 82%]
tests\trial_tests\test_trials.py ....................................... [ 83%]
...                                                                      [ 83%]
tests\visualization_tests\test_contour.py ..
Error: The operation was canceled.

Steps to reproduce

Trigger the windows CI by workflow dispatcher on your fork.

Additional context (optional)

No response