Test to guard against re-introducing #64.

Author: samcunliffe

src/napari_matplotlib/tests/baseline/test_no_theme_side_effects.png

@@ -1,3 +1,5 @@
+from copy import deepcopy
+
 import napari
 import numpy as np
 import pytest
@@ -88,3 +90,31 @@ def test_titles_respect_theme(
 
     assert ax.xaxis.label.get_color() == expected_text_colour
     assert ax.yaxis.label.get_color() == expected_text_colour
+
+
+@pytest.mark.mpl_image_compare
+def test_no_theme_side_effects(make_napari_viewer):
+    """Ensure that napari-matplotlib doesn't pollute the globally set style.
+
+    A MWE to guard aganst issue matplotlib/#64. Should always reproduce a plot
+    with the default matplotlib style.
+    """
+    import matplotlib.pyplot as plt
+
+    np.random.seed(12345)
+
+    # should not affect global matplotlib plot style
+    viewer = make_napari_viewer()
+    viewer.theme = "dark"
+    NapariMPLWidget(viewer)
+
+    # some plotting unrelated to napari-matplotlib
+    image = np.random.random((3, 3))
+    plt.imshow(image)
+    plt.xlabel("something unrelated to napari (x)")
+    plt.ylabel("something unrelated to napari (y)")
+    plt.title("this plot style should not change with napari styles or themes")
+    plt.tight_layout()
+    unrelated_figure = plt.gcf()
+
+    return deepcopy(unrelated_figure)