Using a current install of Python and notebook, matplotlib, ipympl etc. on Windows 11 within VSCode,

%matplotlib ipympl
#%matplotlib widget
import matplotlib.pyplot as plt

and

fig, ax = plt.subplots()
...
ax.plot(...)

raise KeyError: 'buttons' in c:\Users\user\AppData\Local\Programs\Python\Python313\Lib\site-packages\matplotlib\backends\backend_webagg_core.py:295

respectively

Changing the key to button resolves the error (for my use case).