Closes #14396.

The baseline image change was due to previously off-by-one clipping, as
can be checked by additionally drawing a line with the same clipbox and
checking that the image was clipped one pixel row/column too far:

from matplotlib import pyplot as plt, patches

fig, ax = plt.subplots()
d = [[1, 2], [3, 4]]

im = ax.imshow(d, extent=(0, 5, 0, 5), cmap="viridis")
l, = ax.plot([0, 5], [0, 5])

rect = patches.Rectangle(
    xy=(1, 1), width=2, height=2, transform=im.axes.transData)
im.set_clip_path(rect)
l.set_clip_path(rect)

plt.show()

PR Summary

PR Checklist

• Has Pytest style unit tests
• Code is Flake 8 compliant
• New features are documented, with examples if plot related
• Documentation is sphinx and numpydoc compliant
• Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way