From d3d72fc07a35429610afee2ffacd029610627312 Mon Sep 17 00:00:00 2001
From: Tim Hoffmann <2836374+timhoffm@users.noreply.github.com>
Date: Mon, 28 Apr 2025 22:25:30 +0200
Subject: [PATCH 1/5] FIX: Move all tests using subprocess to the same worker
 on windows

This is a somewhat wild guess based on
https://github.com/matplotlib/matplotlib/issues/29797#issuecomment-2833795708

> which makes me think we are somehow crossing state what launching the subprocesses.

Using the `xdist_group` with `--dist=loadgroup` should put all tests of
that group to the same worker according to
https://pytest-xdist.readthedocs.io/en/stable/distribution.html.
I've only added `--dist=loadgroup` to the windows pipelines, so tests on
other systems are not affected at all.

The first test is to see whether this works in the PR. - But to be sure,
it think we would need to put this on master and monitor whether the
timeouts disappear.
---
 azure-pipelines.yml                               | 1 +
 lib/matplotlib/tests/test_backend_inline.py       | 1 +
 lib/matplotlib/tests/test_backend_macosx.py       | 4 ++++
 lib/matplotlib/tests/test_backend_nbagg.py        | 1 +
 lib/matplotlib/tests/test_backend_tk.py           | 1 +
 lib/matplotlib/tests/test_backend_webagg.py       | 1 +
 lib/matplotlib/tests/test_backends_interactive.py | 1 +
 lib/matplotlib/tests/test_basic.py                | 3 +++
 lib/matplotlib/tests/test_determinism.py          | 2 ++
 lib/matplotlib/tests/test_font_manager.py         | 2 ++
 lib/matplotlib/tests/test_matplotlib.py           | 3 +++
 lib/matplotlib/tests/test_pickle.py               | 2 ++
 lib/matplotlib/tests/test_preprocess_data.py      | 1 +
 lib/matplotlib/tests/test_pyplot.py               | 2 ++
 lib/matplotlib/tests/test_rcparams.py             | 3 +++
 lib/matplotlib/tests/test_sphinxext.py            | 5 +++++
 lib/matplotlib/tests/test_texmanager.py           | 1 +
 lib/matplotlib/tests/test_ticker.py               | 1 +
 18 files changed, 35 insertions(+)

diff --git a/azure-pipelines.yml b/azure-pipelines.yml
index cc0fbce11377..3e43de70d485 100644
--- a/azure-pipelines.yml
+++ b/azure-pipelines.yml
@@ -149,6 +149,7 @@ stages:
 
               PYTHONFAULTHANDLER=1 pytest -rfEsXR -n 2 \
                   --maxfail=50 --timeout=300 --durations=25 \
+                  --dist=loadgroup \
                   --junitxml=junit/test-results.xml --cov-report=xml --cov=lib
 
               if [[ $VS_VER == 2022 ]]; then
diff --git a/lib/matplotlib/tests/test_backend_inline.py b/lib/matplotlib/tests/test_backend_inline.py
index 6f0d67d51756..cdd73d9c9c9f 100644
--- a/lib/matplotlib/tests/test_backend_inline.py
+++ b/lib/matplotlib/tests/test_backend_inline.py
@@ -12,6 +12,7 @@
 pytest.importorskip('matplotlib_inline')
 
 
+@pytest.mark.xdist_group(name="subprocess")
 def test_ipynb():
     nb_path = Path(__file__).parent / 'test_inline_01.ipynb'
 
diff --git a/lib/matplotlib/tests/test_backend_macosx.py b/lib/matplotlib/tests/test_backend_macosx.py
index fe4c9a6fba3c..93cad9c21a1d 100644
--- a/lib/matplotlib/tests/test_backend_macosx.py
+++ b/lib/matplotlib/tests/test_backend_macosx.py
@@ -25,6 +25,7 @@ def _test_cached_renderer():
 
 
 @pytest.mark.backend('macosx', skip_on_importerror=True)
+@pytest.mark.xdist_group(name="subprocess")
 def test_cached_renderer():
     subprocess_run_helper(_test_cached_renderer, timeout=_test_timeout,
                           extra_env={"MPLBACKEND": "macosx"})
@@ -55,6 +56,7 @@ def new_choose_save_file(title, directory, filename):
 
 
 @pytest.mark.backend('macosx', skip_on_importerror=True)
+@pytest.mark.xdist_group(name="subprocess")
 def test_savefig_rcparam(tmp_path):
     subprocess_run_helper(
         _test_savefig_rcparam, timeout=_test_timeout,
@@ -62,6 +64,7 @@ def test_savefig_rcparam(tmp_path):
 
 
 @pytest.mark.backend('macosx', skip_on_importerror=True)
+@pytest.mark.xdist_group(name="subprocess")
 def test_ipython():
     from matplotlib.testing import ipython_in_subprocess
     ipython_in_subprocess("osx", {(8, 24): "macosx", (7, 0): "MacOSX"})
@@ -81,6 +84,7 @@ def _test_save_figure_return():
 
 
 @pytest.mark.backend('macosx', skip_on_importerror=True)
+@pytest.mark.xdist_group(name="subprocess")
 def test_save_figure_return():
     subprocess_run_helper(_test_save_figure_return, timeout=_test_timeout,
                           extra_env={"MPLBACKEND": "macosx"})
diff --git a/lib/matplotlib/tests/test_backend_nbagg.py b/lib/matplotlib/tests/test_backend_nbagg.py
index 23af88d95086..ea3a2ea9996c 100644
--- a/lib/matplotlib/tests/test_backend_nbagg.py
+++ b/lib/matplotlib/tests/test_backend_nbagg.py
@@ -13,6 +13,7 @@
 # From https://blog.thedataincubator.com/2016/06/testing-jupyter-notebooks/
 
 
+@pytest.mark.xdist_group(name="subprocess")
 def test_ipynb():
     nb_path = Path(__file__).parent / 'test_nbagg_01.ipynb'
 
diff --git a/lib/matplotlib/tests/test_backend_tk.py b/lib/matplotlib/tests/test_backend_tk.py
index 1210c8c9993e..ff48f5b9bd02 100644
--- a/lib/matplotlib/tests/test_backend_tk.py
+++ b/lib/matplotlib/tests/test_backend_tk.py
@@ -43,6 +43,7 @@ def _isolated_tk_test(success_count, func=None):
         sys.platform == 'darwin' and sys.version_info[:2] < (3, 11),
         reason='Tk version mismatch on Azure macOS CI'
     )
+    @pytest.mark.xdist_group(name="subprocess")
     @functools.wraps(func)
     def test_func():
         # even if the package exists, may not actually be importable this can
diff --git a/lib/matplotlib/tests/test_backend_webagg.py b/lib/matplotlib/tests/test_backend_webagg.py
index 1d6769494ef9..7df12240618a 100644
--- a/lib/matplotlib/tests/test_backend_webagg.py
+++ b/lib/matplotlib/tests/test_backend_webagg.py
@@ -7,6 +7,7 @@
 
 
 @pytest.mark.parametrize("backend", ["webagg", "nbagg"])
+@pytest.mark.xdist_group(name="subprocess")
 def test_webagg_fallback(backend):
     pytest.importorskip("tornado")
     if backend == "nbagg":
diff --git a/lib/matplotlib/tests/test_backends_interactive.py b/lib/matplotlib/tests/test_backends_interactive.py
index a27783fa4be1..149ca96f8372 100644
--- a/lib/matplotlib/tests/test_backends_interactive.py
+++ b/lib/matplotlib/tests/test_backends_interactive.py
@@ -636,6 +636,7 @@ def _fallback_check():
     matplotlib.pyplot.figure()
 
 
+@pytest.mark.xdist_group(name="subprocess")
 def test_fallback_to_different_backend():
     pytest.importorskip("IPython")
     # Runs the process that caused the GH issue 23770
diff --git a/lib/matplotlib/tests/test_basic.py b/lib/matplotlib/tests/test_basic.py
index f6aa1e458555..5db2ede9733f 100644
--- a/lib/matplotlib/tests/test_basic.py
+++ b/lib/matplotlib/tests/test_basic.py
@@ -3,6 +3,8 @@
 import sys
 import textwrap
 
+import pytest
+
 from matplotlib.testing import subprocess_run_for_testing
 
 
@@ -28,6 +30,7 @@ def test_override_builtins():
     assert overridden <= ok_to_override
 
 
+@pytest.mark.xdist_group(name="subprocess")
 def test_lazy_imports():
     source = textwrap.dedent("""
     import sys
diff --git a/lib/matplotlib/tests/test_determinism.py b/lib/matplotlib/tests/test_determinism.py
index 2ecc40dbd3c0..4e713e24e67c 100644
--- a/lib/matplotlib/tests/test_determinism.py
+++ b/lib/matplotlib/tests/test_determinism.py
@@ -154,6 +154,7 @@ def draw(self, renderer=None):
         pytest.param("mhip", "svg", True, marks=needs_usetex),
     ]
 )
+@pytest.mark.xdist_group(name="subprocess")
 def test_determinism_check(objects, fmt, usetex):
     """
     Output three times the same graphs and checks that the outputs are exactly
@@ -195,6 +196,7 @@ def test_determinism_check(objects, fmt, usetex):
         ("ps", b"%%CreationDate: Sat Jan 01 00:00:00 2000"),
     ]
 )
+@pytest.mark.xdist_group(name="subprocess")
 def test_determinism_source_date_epoch(fmt, string):
     """
     Test SOURCE_DATE_EPOCH support. Output a document with the environment
diff --git a/lib/matplotlib/tests/test_font_manager.py b/lib/matplotlib/tests/test_font_manager.py
index d15b892b3eea..3ae2d8bd7100 100644
--- a/lib/matplotlib/tests/test_font_manager.py
+++ b/lib/matplotlib/tests/test_font_manager.py
@@ -282,12 +282,14 @@ def bad_idea(n):
             raise RuntimeError("thread failed to join")
 
 
+@pytest.mark.xdist_group(name="subprocess")
 def test_fontcache_thread_safe():
     pytest.importorskip('threading')
 
     subprocess_run_helper(_test_threading, timeout=10)
 
 
+@pytest.mark.xdist_group(name="subprocess")
 def test_lockfilefailure(tmp_path):
     # The logic here:
     # 1. get a temp directory from pytest
diff --git a/lib/matplotlib/tests/test_matplotlib.py b/lib/matplotlib/tests/test_matplotlib.py
index 37b41fafdb78..863c7c350c7f 100644
--- a/lib/matplotlib/tests/test_matplotlib.py
+++ b/lib/matplotlib/tests/test_matplotlib.py
@@ -22,6 +22,7 @@ def test_parse_to_version_info(version_str, version_tuple):
                     reason="chmod() doesn't work as is on Windows")
 @pytest.mark.skipif(sys.platform != "win32" and os.geteuid() == 0,
                     reason="chmod() doesn't work as root")
+@pytest.mark.xdist_group(name="subprocess")
 def test_tmpconfigdir_warning(tmp_path):
     """Test that a warning is emitted if a temporary configdir must be used."""
     mode = os.stat(tmp_path).st_mode
@@ -36,6 +37,7 @@ def test_tmpconfigdir_warning(tmp_path):
         os.chmod(tmp_path, mode)
 
 
+@pytest.mark.xdist_group(name="subprocess")
 def test_importable_with_no_home(tmp_path):
     subprocess_run_for_testing(
         [sys.executable, "-c",
@@ -65,6 +67,7 @@ def parse(key):
             set(backend_registry.list_builtin(BackendFilter.NON_INTERACTIVE)))
 
 
+@pytest.mark.xdist_group(name="subprocess")
 def test_importable_with__OO():
     """
     When using -OO or export PYTHONOPTIMIZE=2, docstrings are discarded,
diff --git a/lib/matplotlib/tests/test_pickle.py b/lib/matplotlib/tests/test_pickle.py
index 82fc60e186c7..e9094d30c953 100644
--- a/lib/matplotlib/tests/test_pickle.py
+++ b/lib/matplotlib/tests/test_pickle.py
@@ -136,6 +136,7 @@ def _pickle_load_subprocess():
 
 
 @mpl.style.context("default")
+@pytest.mark.xdist_group(name="subprocess")
 @check_figures_equal()
 def test_pickle_load_from_subprocess(fig_test, fig_ref, tmp_path):
     _generate_complete_test_figure(fig_ref)
@@ -331,6 +332,7 @@ def _test_axeswidget_interactive():
         sys.platform == 'darwin' and sys.version_info[:2] < (3, 11),
         reason='Tk version mismatch on Azure macOS CI'
     )
+@pytest.mark.xdist_group(name="subprocess")
 def test_axeswidget_interactive():
     subprocess_run_helper(
         _test_axeswidget_interactive,
diff --git a/lib/matplotlib/tests/test_preprocess_data.py b/lib/matplotlib/tests/test_preprocess_data.py
index c983d78786e1..fd65d1694f5e 100644
--- a/lib/matplotlib/tests/test_preprocess_data.py
+++ b/lib/matplotlib/tests/test_preprocess_data.py
@@ -245,6 +245,7 @@ def funcy(ax, x, y, z, t=None):
                          funcy.__doc__)
 
 
+@pytest.mark.xdist_group(name="subprocess")
 def test_data_parameter_replacement():
     """
     Test that the docstring contains the correct *data* parameter stub
diff --git a/lib/matplotlib/tests/test_pyplot.py b/lib/matplotlib/tests/test_pyplot.py
index ab713707bace..db68fbfa0c01 100644
--- a/lib/matplotlib/tests/test_pyplot.py
+++ b/lib/matplotlib/tests/test_pyplot.py
@@ -11,6 +11,7 @@
 from matplotlib import pyplot as plt
 
 
+@pytest.mark.xdist_group(name="subprocess")
 def test_pyplot_up_to_date(tmp_path):
     pytest.importorskip("black")
 
@@ -348,6 +349,7 @@ def test_set_current_axes_on_subfigure():
     assert plt.gca() == ax
 
 
+@pytest.mark.xdist_group(name="subprocess")
 def test_pylab_integration():
     IPython = pytest.importorskip("IPython")
     mpl.testing.subprocess_run_helper(
diff --git a/lib/matplotlib/tests/test_rcparams.py b/lib/matplotlib/tests/test_rcparams.py
index 1bc148a83a7e..a52c52d3a4f6 100644
--- a/lib/matplotlib/tests/test_rcparams.py
+++ b/lib/matplotlib/tests/test_rcparams.py
@@ -529,6 +529,7 @@ def test_rcparams_reset_after_fail():
 
 
 @pytest.mark.skipif(sys.platform != "linux", reason="Linux only")
+@pytest.mark.xdist_group(name="subprocess")
 def test_backend_fallback_headless_invalid_backend(tmp_path):
     env = {**os.environ,
            "DISPLAY": "", "WAYLAND_DISPLAY": "",
@@ -546,6 +547,7 @@ def test_backend_fallback_headless_invalid_backend(tmp_path):
 
 
 @pytest.mark.skipif(sys.platform != "linux", reason="Linux only")
+@pytest.mark.xdist_group(name="subprocess")
 def test_backend_fallback_headless_auto_backend(tmp_path):
     # specify a headless mpl environment, but request a graphical (tk) backend
     env = {**os.environ,
@@ -570,6 +572,7 @@ def test_backend_fallback_headless_auto_backend(tmp_path):
 @pytest.mark.skipif(
     sys.platform == "linux" and not _c_internal_utils.xdisplay_is_valid(),
     reason="headless")
+@pytest.mark.xdist_group(name="subprocess")
 def test_backend_fallback_headful(tmp_path):
     if parse_version(pytest.__version__) >= parse_version('8.2.0'):
         pytest_kwargs = dict(exc_type=ImportError)
diff --git a/lib/matplotlib/tests/test_sphinxext.py b/lib/matplotlib/tests/test_sphinxext.py
index 1aaa6baca47c..816cc01a4e03 100644
--- a/lib/matplotlib/tests/test_sphinxext.py
+++ b/lib/matplotlib/tests/test_sphinxext.py
@@ -32,6 +32,7 @@ def build_sphinx_html(source_dir, doctree_dir, html_dir, extra_args=None):
     assert html_dir.is_dir()
 
 
+@pytest.mark.xdist_group(name="subprocess")
 def test_tinypages(tmp_path):
     shutil.copytree(Path(__file__).parent / 'tinypages', tmp_path,
                     dirs_exist_ok=True)
@@ -124,6 +125,7 @@ def plot_directive_file(num):
     assert filecmp.cmp(range_6, plot_file(5))
 
 
+@pytest.mark.xdist_group(name="subprocess")
 def test_plot_html_show_source_link(tmp_path):
     parent = Path(__file__).parent
     shutil.copyfile(parent / 'tinypages/conf.py', tmp_path / 'conf.py')
@@ -147,6 +149,7 @@ def test_plot_html_show_source_link(tmp_path):
 
 
 @pytest.mark.parametrize('plot_html_show_source_link', [0, 1])
+@pytest.mark.xdist_group(name="subprocess")
 def test_show_source_link_true(tmp_path, plot_html_show_source_link):
     # Test that a source link is generated if :show-source-link: is true,
     # whether or not plot_html_show_source_link is true.
@@ -167,6 +170,7 @@ def test_show_source_link_true(tmp_path, plot_html_show_source_link):
 
 
 @pytest.mark.parametrize('plot_html_show_source_link', [0, 1])
+@pytest.mark.xdist_group(name="subprocess")
 def test_show_source_link_false(tmp_path, plot_html_show_source_link):
     # Test that a source link is NOT generated if :show-source-link: is false,
     # whether or not plot_html_show_source_link is true.
@@ -186,6 +190,7 @@ def test_show_source_link_false(tmp_path, plot_html_show_source_link):
     assert len(list(html_dir.glob("**/index-1.py"))) == 0
 
 
+@pytest.mark.xdist_group(name="subprocess")
 def test_srcset_version(tmp_path):
     shutil.copytree(Path(__file__).parent / 'tinypages', tmp_path,
                     dirs_exist_ok=True)
diff --git a/lib/matplotlib/tests/test_texmanager.py b/lib/matplotlib/tests/test_texmanager.py
index 64dcbf46456d..bf67ca52fcf2 100644
--- a/lib/matplotlib/tests/test_texmanager.py
+++ b/lib/matplotlib/tests/test_texmanager.py
@@ -64,6 +64,7 @@ def test_unicode_characters():
 
 
 @needs_usetex
+@pytest.mark.xdist_group(name="subprocess")
 def test_openin_any_paranoid():
     completed = subprocess_run_for_testing(
         [sys.executable, "-c",
diff --git a/lib/matplotlib/tests/test_ticker.py b/lib/matplotlib/tests/test_ticker.py
index 0f54230663aa..17fc961220e3 100644
--- a/lib/matplotlib/tests/test_ticker.py
+++ b/lib/matplotlib/tests/test_ticker.py
@@ -1766,6 +1766,7 @@ def _impl_locale_comma():
     assert x == '0,5'
 
 
+@pytest.mark.xdist_group(name="subprocess")
 def test_locale_comma():
     # On some systems/pytest versions, `pytest.skip` in an exception handler
     # does not skip, but is treated as an exception, so directly running this

From 75ba0d53b260011d8690d999cc63ac441b5afc8b Mon Sep 17 00:00:00 2001
From: Thomas A Caswell <tcaswell@gmail.com>
Date: Tue, 29 Apr 2025 09:21:05 -0400
Subject: [PATCH 2/5] TST: merge tests that share the same baseline image

---
 lib/matplotlib/tests/test_patches.py | 42 ++++++++++------------------
 1 file changed, 14 insertions(+), 28 deletions(-)

diff --git a/lib/matplotlib/tests/test_patches.py b/lib/matplotlib/tests/test_patches.py
index 4ed9222eb95e..b0246d669eeb 100644
--- a/lib/matplotlib/tests/test_patches.py
+++ b/lib/matplotlib/tests/test_patches.py
@@ -825,8 +825,9 @@ def test_annulus():
     ax.set_aspect('equal')
 
 
+@pytest.mark.parametrize('mode', ('a', 'b'))
 @image_comparison(baseline_images=['annulus'], extensions=['png'])
-def test_annulus_setters():
+def test_annulus_setters(mode):
 
     fig, ax = plt.subplots()
     cir = Annulus((0., 0.), 0.2, 0.01, fc='g')   # circular annulus
@@ -837,36 +838,21 @@ def test_annulus_setters():
     ax.set_aspect('equal')
 
     cir.center = (0.5, 0.5)
-    cir.radii = 0.2
+    if mode == 'a':
+        cir.set_semimajor(0.2)
+        cir.set_semiminor(0.2)
+        assert cir.radii == (0.2, 0.2)
+    elif mode == 'b':
+        cir.radii = 0.2
     cir.width = 0.05
 
     ell.center = (0.5, 0.5)
-    ell.radii = (0.5, 0.3)
-    ell.width = 0.1
-    ell.angle = 45
-
-
-@image_comparison(baseline_images=['annulus'], extensions=['png'])
-def test_annulus_setters2():
-
-    fig, ax = plt.subplots()
-    cir = Annulus((0., 0.), 0.2, 0.01, fc='g')   # circular annulus
-    ell = Annulus((0., 0.), (1, 2), 0.1, 0,      # elliptical
-                  fc='m', ec='b', alpha=0.5, hatch='xxx')
-    ax.add_patch(cir)
-    ax.add_patch(ell)
-    ax.set_aspect('equal')
-
-    cir.center = (0.5, 0.5)
-    cir.set_semimajor(0.2)
-    cir.set_semiminor(0.2)
-    assert cir.radii == (0.2, 0.2)
-    cir.width = 0.05
-
-    ell.center = (0.5, 0.5)
-    ell.set_semimajor(0.5)
-    ell.set_semiminor(0.3)
-    assert ell.radii == (0.5, 0.3)
+    if mode == 'a':
+        ell.set_semimajor(0.5)
+        ell.set_semiminor(0.3)
+        assert ell.radii == (0.5, 0.3)
+    elif mode == 'b':
+        ell.radii = (0.5, 0.3)
     ell.width = 0.1
     ell.angle = 45
 

From dc401b37d94a8ae19de8f405e7b35da1ada272c5 Mon Sep 17 00:00:00 2001
From: Thomas A Caswell <tcaswell@gmail.com>
Date: Tue, 29 Apr 2025 21:54:12 -0400
Subject: [PATCH 3/5] TST: re-factor a bit and use better parameter names

---
 lib/matplotlib/tests/test_patches.py | 17 +++++++++--------
 1 file changed, 9 insertions(+), 8 deletions(-)

diff --git a/lib/matplotlib/tests/test_patches.py b/lib/matplotlib/tests/test_patches.py
index b0246d669eeb..9771db03e330 100644
--- a/lib/matplotlib/tests/test_patches.py
+++ b/lib/matplotlib/tests/test_patches.py
@@ -825,7 +825,7 @@ def test_annulus():
     ax.set_aspect('equal')
 
 
-@pytest.mark.parametrize('mode', ('a', 'b'))
+@pytest.mark.parametrize('mode', ('by_semiaxis', 'by_radius'))
 @image_comparison(baseline_images=['annulus'], extensions=['png'])
 def test_annulus_setters(mode):
 
@@ -838,22 +838,23 @@ def test_annulus_setters(mode):
     ax.set_aspect('equal')
 
     cir.center = (0.5, 0.5)
-    if mode == 'a':
+    ell.center = (0.5, 0.5)
+
+    if mode == 'by_semiaxis':
         cir.set_semimajor(0.2)
         cir.set_semiminor(0.2)
         assert cir.radii == (0.2, 0.2)
-    elif mode == 'b':
-        cir.radii = 0.2
-    cir.width = 0.05
 
-    ell.center = (0.5, 0.5)
-    if mode == 'a':
         ell.set_semimajor(0.5)
         ell.set_semiminor(0.3)
         assert ell.radii == (0.5, 0.3)
-    elif mode == 'b':
+    elif mode == 'by_radius':
+        cir.radii = 0.2
         ell.radii = (0.5, 0.3)
+
+    cir.width = 0.05
     ell.width = 0.1
+
     ell.angle = 45
 
 

From 13d1e51238225e9efdffbe5dcd35767f6ef775c2 Mon Sep 17 00:00:00 2001
From: Tim Hoffmann <2836374+timhoffm@users.noreply.github.com>
Date: Wed, 30 Apr 2025 13:05:17 +0200
Subject: [PATCH 4/5] Limit usage of xdist grouping to Python 3.11

---
 azure-pipelines.yml | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/azure-pipelines.yml b/azure-pipelines.yml
index 3e43de70d485..9b325a683209 100644
--- a/azure-pipelines.yml
+++ b/azure-pipelines.yml
@@ -147,9 +147,13 @@ stages:
               fi
               echo "##vso[task.setvariable variable=VS_COVERAGE_TOOL]$TOOL"
 
+              if [[ $(python.version) == 3.11 ]]; then
+                # when removing this, also remove
+                # @pytest.mark.xdist_group(name="subprocess") from tests
+                DIST_LOAD_GROUP=--dist=loadgroup
+              fi
               PYTHONFAULTHANDLER=1 pytest -rfEsXR -n 2 \
-                  --maxfail=50 --timeout=300 --durations=25 \
-                  --dist=loadgroup \
+                  --maxfail=50 --timeout=300 --durations=25 $DIST_LOAD_GROUP \
                   --junitxml=junit/test-results.xml --cov-report=xml --cov=lib
 
               if [[ $VS_VER == 2022 ]]; then

From 0590a05fdd7c5226a32540186335f60eed48e409 Mon Sep 17 00:00:00 2001
From: Tim Hoffmann <2836374+timhoffm@users.noreply.github.com>
Date: Thu, 1 May 2025 10:03:44 +0200
Subject: [PATCH 5/5] Add some more tests using subprocesses to the subprocess
 xdist_group

Note: Some of them have been marked flaky - may be worth reconstructing
whether that was due to timeouts.
---
 lib/matplotlib/tests/test_backends_interactive.py | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/lib/matplotlib/tests/test_backends_interactive.py b/lib/matplotlib/tests/test_backends_interactive.py
index 149ca96f8372..d9c26378d75d 100644
--- a/lib/matplotlib/tests/test_backends_interactive.py
+++ b/lib/matplotlib/tests/test_backends_interactive.py
@@ -238,6 +238,7 @@ def check_alt_backend(alt_backend):
 @pytest.mark.parametrize("env", _get_testable_interactive_backends())
 @pytest.mark.parametrize("toolbar", ["toolbar2", "toolmanager"])
 @pytest.mark.flaky(reruns=3)
+@pytest.mark.xdist_group(name="subprocess")
 def test_interactive_backend(env, toolbar):
     if env["MPLBACKEND"] == "macosx":
         if toolbar == "toolmanager":
@@ -330,6 +331,7 @@ def _test_thread_impl():
 
 @pytest.mark.parametrize("env", _thread_safe_backends)
 @pytest.mark.flaky(reruns=3)
+@pytest.mark.xdist_group(name="subprocess")
 def test_interactive_thread_safety(env):
     proc = _run_helper(_test_thread_impl, timeout=_test_timeout, extra_env=env)
     assert proc.stdout.count("CloseEvent") == 1
@@ -349,6 +351,7 @@ def _impl_test_lazy_auto_backend_selection():
     assert isinstance(bk, str)
 
 
+@pytest.mark.xdist_group(name="subprocess")
 def test_lazy_auto_backend_selection():
     _run_helper(_impl_test_lazy_auto_backend_selection,
                 timeout=_test_timeout)
@@ -381,6 +384,7 @@ def _implcore():
     assert 'PyQt5' in sys.modules or 'pyside2' in sys.modules
 
 
+@pytest.mark.xdist_group(name="subprocess")
 def test_qt5backends_uses_qt5():
     qt5_bindings = [
         dep for dep in ['PyQt5', 'pyside2']
@@ -414,6 +418,7 @@ def _impl_missing():
         plt.switch_backend("qt5agg")
 
 
+@pytest.mark.xdist_group(name="subprocess")
 def test_qt_missing():
     _run_helper(_impl_missing, timeout=_test_timeout)
 
@@ -459,6 +464,7 @@ def qt5_and_qt6_pairs():
     sys.platform == "linux" and not _c_internal_utils.display_is_valid(),
     reason="$DISPLAY and $WAYLAND_DISPLAY are unset")
 @pytest.mark.parametrize('host, mpl', [*qt5_and_qt6_pairs()])
+@pytest.mark.xdist_group(name="subprocess")
 def test_cross_Qt_imports(host, mpl):
     try:
         proc = _run_helper(_impl_test_cross_Qt_imports, host, mpl,
@@ -541,6 +547,7 @@ def _lazy_headless():
 
 @pytest.mark.skipif(sys.platform != "linux", reason="this a linux-only test")
 @pytest.mark.parametrize("env", _get_testable_interactive_backends())
+@pytest.mark.xdist_group(name="subprocess")
 def test_lazy_linux_headless(env):
     proc = _run_helper(
         _lazy_headless,
@@ -618,6 +625,7 @@ def _test_number_of_draws_script():
 @pytest.mark.parametrize("env", _blit_backends)
 # subprocesses can struggle to get the display, so rerun a few times
 @pytest.mark.flaky(reruns=4)
+@pytest.mark.xdist_group(name="subprocess")
 def test_blitting_events(env):
     proc = _run_helper(
         _test_number_of_draws_script, timeout=_test_timeout, extra_env=env)
@@ -681,6 +689,7 @@ def _impl_test_interactive_timers():
 
 
 @pytest.mark.parametrize("env", _get_testable_interactive_backends())
+@pytest.mark.xdist_group(name="subprocess")
 def test_interactive_timers(env):
     if env["MPLBACKEND"] == "gtk3cairo" and os.getenv("CI"):
         pytest.skip("gtk3cairo timers do not work in remote CI")