PR summary

This PR adds a new test to verify that changing the colormap of a 3D scatter plot (Axes3D.scatter) after drawing the figure properly updates the face colors.

Why is this change necessary?

There was previously no test ensuring that colormap updates on 3D scatter plots are reflected after a figure draw. This test ensures consistency with 2D behavior and guards against regressions.

What problem does it solve?

Confirms that calling set_cmap() after drawing does in fact update the plotted colors in 3D, which has historically been a bug but seems to be fixed.

What is the reasoning for this implementation?

The test draws a 3D scatter plot with an initial colormap ('jet'), forces a draw, changes the colormap to 'viridis', and compares it against a reference plot created directly with 'viridis'.

This ensures the update mechanism behaves correctly post-render.

Minimal example

sc = ax.scatter(x, y, z, c=values, cmap='jet')
fig.canvas.draw()
sc.set_cmap('viridis')

Closes #18931

PR checklist

• "closes 3D collections do not proper handle edgecolor='face' #18931" 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