Fix a bug of ignoring the closing path segment when automatically determining whether to snap - #32018

PR summary

The current code for automatic determination of whether a path should be snapped – by checking whether all line segments are either horizontal or vertical – neglects to check the closing line segment for a closed path. That is, a closed path will be automatically determined to be snappable even when the closing line segment is not horizontal or vertical as long as all other line segments are either horizontal or vertical. This PR fixes that oversight.

A not-uncommon place for this bug to have an effect in the wild is when plotting contours, and one contour region is just a triangular section at the corner of the plot. If the diagonal of that triangle happens to be the closing line segment, then that contour region will snap, while its adjacent contour region will typically not snap. A handful of figure tests show this effect.

Here is a minimal example illustrating the bug:

import matplotlib.pyplot as plt
from matplotlib.patches import Polygon

# diagonal is not the closing segment, so will not be snapped
triangle1 = Polygon([[0, 0], [1, 0], [0, 1]])

# diagonal is the closing segment, so will be snapped due to the bug
triangle2 = Polygon([[0, 1], [1, 1], [1, 0]])

fig, ax = plt.subplots()

ax.add_patch(triangle1)
ax.add_patch(triangle2)

plt.show()

Before this PR

The gap between the two triangles is not a consistent width, showing that the two triangles are not snapping the same. The gap will change/flicker if the plot window is resized due to this difference in snapping.

After this PR

The gap between the two triangles is a consistent width, showing that both triangles are snapping the same.

AI Disclosure

No AI was used

PR checklist

• [N/A] "closes #0000" is in the body of the PR description to link the related issue
• new and changed code is tested
• [N/A] Plotting related features are demonstrated in an example
• [N/A] New Features and API Changes are noted with a directive and release note
• [N/A] Documentation complies with general and docstring guidelines