tripcolor shading="gouraud" streaks/glitch on macOS arm64; shading="flat" is fine #32269

Bug summary

On macOS arm64, tripcolor(..., shading="gouraud") draws long coloured streaks and glitches. The same mesh with shading="flat" is fine. Standalone matplotlib + numpy, Agg backend. Other machines render both panels identically.

Code for reproduction

import matplotlib.pyplot as plt
import numpy as np

m = np.load("gouraud_mesh.npz")  # attach this file
pts, cells, vals = m["pts"], m["cells"], m["vals"]
fig, ax = plt.subplots(1, 2, figsize=(12, 4), constrained_layout=True)
for a, sh in zip(ax, ("flat", "gouraud")):
    im = a.tripcolor(pts[:, 0], pts[:, 1], cells, vals, shading=sh, cmap="jet")
    a.set_aspect("equal")
    a.set_title(sh)
    fig.colorbar(im, ax=a)
fig.savefig("gouraud_test_result.png", dpi=200)

Actual outcome

Left (flat): smooth field. Right (gouraud): radial streaks (PNG attached). Same with savefig/Agg.

Expected outcome

Gouraud should look like a smooth version of the flat panel, matching other machines.

Additional information

matplotlib 3.10.3 backend: Agg
numpy 2.0.2
python 3.12.7
OS macOS-26.6.2-arm64-arm-64bit

Also on matplotlib 3.11.1

Only the Gouraud path fails. Force-reinstall and a fresh env did not help. Workaround: shading="flat". Distinct from #28769 (small colour offset, not streaking).

Operating system

macOS 26.6.2 arm64

Matplotlib Version

3.10.3 (also 3.11.1)

Matplotlib Backend

Agg (also macosx)

Python version

3.12.7

Jupyter version

No response

Installation

pip