PR Summary

Closing a window would keep the window around with a spinning ball. If we flush the pending events, the window can close immediately.

import numpy as np
import matplotlib.pyplot as plt
import time

x = np.arange(5)
y = np.arange(5)

plt.plot(x, y)
plt.show()

time.sleep(5)

This will need to be tested manually. I'm not sure how we would be able to test it otherwise.

Closes #11861

PR Checklist

Tests and Styling

• [N/A] Has pytest style unit tests (and pytest passes).
• Is Flake 8 compliant (install flake8-docstrings and run flake8 --docstring-convention=all).

Documentation

• [N/A] New features are documented, with examples if plot related.
• [N/A] New features have an entry in doc/users/next_whats_new/ (follow instructions in README.rst there).
• [N/A] API changes documented in doc/api/next_api_changes/ (follow instructions in README.rst there).
• [N/A] Documentation is sphinx and numpydoc compliant (the docs should build without error).