From a866298c33e6b264e2408c018e2eb7e3b3069ee2 Mon Sep 17 00:00:00 2001
From: Elliott Sales de Andrade <quantum.analyst@gmail.com>
Date: Tue, 8 Jul 2025 00:30:51 -0400
Subject: [PATCH 01/11] Fix mlab fallback for 32-bit systems

Unfortunately, I applied the change from
https://github.com/matplotlib/matplotlib/pull/29115#discussion_r2189746839
directly without noticing the typo, or running full tests.

So fix the swapped condition, and add a test (for `csd` only, which
should be enough since everything goes though `_spectral_helper`.)
---
 lib/matplotlib/mlab.py            |  4 ++--
 lib/matplotlib/tests/test_mlab.py | 13 ++++++++++++-
 2 files changed, 14 insertions(+), 3 deletions(-)

diff --git a/lib/matplotlib/mlab.py b/lib/matplotlib/mlab.py
index b4b4c3f96828..4a4480c485fc 100644
--- a/lib/matplotlib/mlab.py
+++ b/lib/matplotlib/mlab.py
@@ -215,9 +215,9 @@ def _stride_windows(x, n, noverlap=0):
     x = np.asarray(x)
 
     _api.check_isinstance(Integral, n=n, noverlap=noverlap)
-    if not (1 <= n <= x.size and n < noverlap):
+    if not (1 <= n <= x.size and noverlap < n):
         raise ValueError(f'n ({n}) and noverlap ({noverlap}) must be positive integers '
-                         f'with n < noverlap and n <= x.size ({x.size})')
+                         f'with noverlap < n and n <= x.size ({x.size})')
 
     if n == 1 and noverlap == 0:
         return x[np.newaxis]
diff --git a/lib/matplotlib/tests/test_mlab.py b/lib/matplotlib/tests/test_mlab.py
index 3b0d2529b5f1..109a6d542450 100644
--- a/lib/matplotlib/tests/test_mlab.py
+++ b/lib/matplotlib/tests/test_mlab.py
@@ -1,3 +1,5 @@
+import sys
+
 from numpy.testing import (assert_allclose, assert_almost_equal,
                            assert_array_equal, assert_array_almost_equal_nulp)
 import numpy as np
@@ -429,7 +431,16 @@ def test_spectral_helper_psd(self, mode, case):
         assert spec.shape[0] == freqs.shape[0]
         assert spec.shape[1] == getattr(self, f"t_{case}").shape[0]
 
-    def test_csd(self):
+    @pytest.mark.parametrize('bitsize', [
+        pytest.param(None, id='default'),
+        pytest.param(32,
+                     marks=pytest.mark.skipif(sys.maxsize <= 2**32,
+                                              reason='System is already 32-bit'),
+                     id='32-bit')
+    ])
+    def test_csd(self, bitsize, monkeypatch):
+        if bitsize is not None:
+            monkeypatch.setattr(sys, 'maxsize', 2**bitsize)
         freqs = self.freqs_density
         spec, fsp = mlab.csd(x=self.y, y=self.y+1,
                              NFFT=self.NFFT_density,

From 8eceb82c94423addd6af231753fac81b57dfcabe Mon Sep 17 00:00:00 2001
From: Elliott Sales de Andrade <quantum.analyst@gmail.com>
Date: Tue, 4 Jun 2024 16:45:34 -0700
Subject: [PATCH 02/11] TST: Fix condition for test_tmpconfigdir_warning

This checks `os.geteuid`, but this is only available on Unix, not
Emscripten or WASM.
---
 lib/matplotlib/tests/test_matplotlib.py | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/lib/matplotlib/tests/test_matplotlib.py b/lib/matplotlib/tests/test_matplotlib.py
index d0a3f8c617e1..312fa4154b66 100644
--- a/lib/matplotlib/tests/test_matplotlib.py
+++ b/lib/matplotlib/tests/test_matplotlib.py
@@ -19,9 +19,9 @@ def test_parse_to_version_info(version_str, version_tuple):
     assert matplotlib._parse_to_version_info(version_str) == version_tuple
 
 
-@pytest.mark.skipif(sys.platform == "win32",
-                    reason="chmod() doesn't work as is on Windows")
-@pytest.mark.skipif(sys.platform != "win32" and os.geteuid() == 0,
+@pytest.mark.skipif(sys.platform not in ["linux", "darwin"],
+                    reason="chmod() doesn't work on this platform")
+@pytest.mark.skipif(sys.platform in ["linux", "darwin"] and os.geteuid() == 0,
                     reason="chmod() doesn't work as root")
 def test_tmpconfigdir_warning(tmp_path):
     """Test that a warning is emitted if a temporary configdir must be used."""

From 5095a3310c666c940ed27f43d1ce7e9e4fc294f9 Mon Sep 17 00:00:00 2001
From: Elliott Sales de Andrade <quantum.analyst@gmail.com>
Date: Tue, 4 Jun 2024 17:15:43 -0700
Subject: [PATCH 03/11] Ignore threading errors when preparing font cache

In Enscripten/WASM, this module can be imported, but doesn't work, so we
can't fall back to `dummy_threading` at import-time as we used to do.
---
 lib/matplotlib/font_manager.py | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/lib/matplotlib/font_manager.py b/lib/matplotlib/font_manager.py
index 2db98b75ab2e..60d2ca0c2a1d 100644
--- a/lib/matplotlib/font_manager.py
+++ b/lib/matplotlib/font_manager.py
@@ -1086,9 +1086,12 @@ def __init__(self, size=None, weight='normal'):
         self.ttflist = []
 
         # Delay the warning by 5s.
-        timer = threading.Timer(5, lambda: _log.warning(
-            'Matplotlib is building the font cache; this may take a moment.'))
-        timer.start()
+        try:
+            timer = threading.Timer(5, lambda: _log.warning(
+                'Matplotlib is building the font cache; this may take a moment.'))
+            timer.start()
+        except RuntimeError:
+            timer = None
         try:
             for fontext in ["afm", "ttf"]:
                 for path in [*findSystemFonts(paths, fontext=fontext),
@@ -1101,7 +1104,8 @@ def __init__(self, size=None, weight='normal'):
                         _log.info("Failed to extract font properties from %s: "
                                   "%s", path, exc)
         finally:
-            timer.cancel()
+            if timer:
+                timer.cancel()
 
     def addfont(self, path):
         """

From 09527b09edebeaf07a8fdf48637521dc67ae66cc Mon Sep 17 00:00:00 2001
From: Elliott Sales de Andrade <quantum.analyst@gmail.com>
Date: Tue, 4 Jun 2024 17:28:28 -0700
Subject: [PATCH 04/11] Don't attempt to load system fonts on Emscripten

The file system is either sandboxed or there are simply no other fonts,
so limit ourselves to our pre-shipped fonts.
---
 lib/matplotlib/font_manager.py            | 3 +++
 lib/matplotlib/tests/test_font_manager.py | 2 +-
 2 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/lib/matplotlib/font_manager.py b/lib/matplotlib/font_manager.py
index 60d2ca0c2a1d..014c6b713fe2 100644
--- a/lib/matplotlib/font_manager.py
+++ b/lib/matplotlib/font_manager.py
@@ -287,6 +287,9 @@ def findSystemFonts(fontpaths=None, fontext='ttf'):
         if sys.platform == 'win32':
             installed_fonts = _get_win32_installed_fonts()
             fontpaths = []
+        elif sys.platform == 'emscripten':
+            installed_fonts = []
+            fontpaths = []
         else:
             installed_fonts = _get_fontconfig_fonts()
             if sys.platform == 'darwin':
diff --git a/lib/matplotlib/tests/test_font_manager.py b/lib/matplotlib/tests/test_font_manager.py
index 97ee8672b1d4..2253596ac3e2 100644
--- a/lib/matplotlib/tests/test_font_manager.py
+++ b/lib/matplotlib/tests/test_font_manager.py
@@ -20,7 +20,7 @@
 from matplotlib.testing import subprocess_run_helper, subprocess_run_for_testing
 
 
-has_fclist = shutil.which('fc-list') is not None
+has_fclist = sys.platform != 'emscripten' and shutil.which('fc-list') is not None
 
 
 def test_font_priority():

From ed9d5ecd677071efa6788f8eb4741fb6cbece37e Mon Sep 17 00:00:00 2001
From: Elliott Sales de Andrade <quantum.analyst@gmail.com>
Date: Thu, 26 Sep 2024 06:14:03 -0400
Subject: [PATCH 05/11] agg: Explicitly cast dimensions when creating Python
 buffer

On WASM, which is wholly 32-bit, casting unsigned int to signed long is
a narrowing conversion, which it seems to treat as an error. The Agg
buffer cannot be over `(1<<23)` in width or height, so this cast is
safe even with the smaller sizes.
---
 src/_backend_agg_wrapper.cpp | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/src/_backend_agg_wrapper.cpp b/src/_backend_agg_wrapper.cpp
index 3dd50b31f64a..11d45773d186 100644
--- a/src/_backend_agg_wrapper.cpp
+++ b/src/_backend_agg_wrapper.cpp
@@ -254,12 +254,12 @@ PYBIND11_MODULE(_backend_agg, m, py::mod_gil_not_used())
 
         .def_buffer([](RendererAgg *renderer) -> py::buffer_info {
             std::vector<py::ssize_t> shape {
-                renderer->get_height(),
-                renderer->get_width(),
+                static_cast<py::ssize_t>(renderer->get_height()),
+                static_cast<py::ssize_t>(renderer->get_width()),
                 4
             };
             std::vector<py::ssize_t> strides {
-                renderer->get_width() * 4,
+                static_cast<py::ssize_t>(renderer->get_width() * 4),
                 4,
                 1
             };

From 616c0bfc50097fa07630b937aa268c034d0356c6 Mon Sep 17 00:00:00 2001
From: Elliott Sales de Andrade <quantum.analyst@gmail.com>
Date: Tue, 5 Nov 2024 04:33:59 -0500
Subject: [PATCH 06/11] wasm: Skip more subprocess & thread tests

---
 lib/matplotlib/animation.py               | 2 ++
 lib/matplotlib/dviread.py                 | 4 ++--
 lib/matplotlib/testing/__init__.py        | 7 ++++++-
 lib/matplotlib/testing/decorators.py      | 2 ++
 lib/matplotlib/tests/test_animation.py    | 4 ++++
 lib/matplotlib/tests/test_dviread.py      | 3 ++-
 lib/matplotlib/tests/test_figure.py       | 5 +++++
 lib/matplotlib/tests/test_font_manager.py | 2 ++
 lib/matplotlib/texmanager.py              | 8 ++++----
 9 files changed, 29 insertions(+), 8 deletions(-)

diff --git a/lib/matplotlib/animation.py b/lib/matplotlib/animation.py
index 8756cb0c1439..7267644b8b19 100644
--- a/lib/matplotlib/animation.py
+++ b/lib/matplotlib/animation.py
@@ -369,6 +369,8 @@ def bin_path(cls):
     @classmethod
     def isAvailable(cls):
         """Return whether a MovieWriter subclass is actually available."""
+        if sys.platform == 'emscripten':
+            return False
         return shutil.which(cls.bin_path()) is not None
 
 
diff --git a/lib/matplotlib/dviread.py b/lib/matplotlib/dviread.py
index 9e8b6a5facf5..20c19e8a5b19 100644
--- a/lib/matplotlib/dviread.py
+++ b/lib/matplotlib/dviread.py
@@ -1135,7 +1135,7 @@ def find_tex_file(filename):
 
     try:
         lk = _LuatexKpsewhich()
-    except FileNotFoundError:
+    except (FileNotFoundError, OSError):
         lk = None  # Fallback to directly calling kpsewhich, as below.
 
     if lk:
@@ -1155,7 +1155,7 @@ def find_tex_file(filename):
             path = (cbook._check_and_log_subprocess(['kpsewhich', filename],
                                                     _log, **kwargs)
                     .rstrip('\n'))
-        except (FileNotFoundError, RuntimeError):
+        except (FileNotFoundError, OSError, RuntimeError):
             path = None
 
     if path:
diff --git a/lib/matplotlib/testing/__init__.py b/lib/matplotlib/testing/__init__.py
index d6affb1b039f..c7319c0aba56 100644
--- a/lib/matplotlib/testing/__init__.py
+++ b/lib/matplotlib/testing/__init__.py
@@ -87,9 +87,14 @@ def subprocess_run_for_testing(command, env=None, timeout=60, stdout=None,
 
     Raises
     ------
+    pytest.skip
+        If running on emscripten, which does not support subprocesses.
     pytest.xfail
         If platform is Cygwin and subprocess reports a fork() failure.
     """
+    if sys.platform == 'emscripten':
+        import pytest
+        pytest.skip('emscripten does not support subprocesses')
     if capture_output:
         stdout = stderr = subprocess.PIPE
     try:
@@ -177,7 +182,7 @@ def _has_tex_package(package):
     try:
         mpl.dviread.find_tex_file(f"{package}.sty")
         return True
-    except FileNotFoundError:
+    except (FileNotFoundError, OSError):
         return False
 
 
diff --git a/lib/matplotlib/testing/decorators.py b/lib/matplotlib/testing/decorators.py
index 17509449e768..f99d8a4866cd 100644
--- a/lib/matplotlib/testing/decorators.py
+++ b/lib/matplotlib/testing/decorators.py
@@ -138,6 +138,8 @@ def copy_baseline(self, baseline, extension):
             try:
                 if 'microsoft' in uname().release.lower():
                     raise OSError  # On WSL, symlink breaks silently
+                if sys.platform == 'emscripten':
+                    raise OSError
                 os.symlink(orig_expected_path, expected_fname)
             except OSError:  # On Windows, symlink *may* be unavailable.
                 shutil.copyfile(orig_expected_path, expected_fname)
diff --git a/lib/matplotlib/tests/test_animation.py b/lib/matplotlib/tests/test_animation.py
index 114e38996a10..b34dc01e41cb 100644
--- a/lib/matplotlib/tests/test_animation.py
+++ b/lib/matplotlib/tests/test_animation.py
@@ -271,6 +271,8 @@ def test_no_length_frames(anim):
     anim.save('unused.null', writer=NullMovieWriter())
 
 
+@pytest.mark.skipif(sys.platform == 'emscripten',
+                    reason='emscripten does not support subprocesses')
 def test_movie_writer_registry():
     assert len(animation.writers._registered) > 0
     mpl.rcParams['animation.ffmpeg_path'] = "not_available_ever_xxxx"
@@ -522,6 +524,8 @@ def test_disable_cache_warning(anim):
 def test_movie_writer_invalid_path(anim):
     if sys.platform == "win32":
         match_str = r"\[WinError 3] .*\\\\foo\\\\bar\\\\aardvark'"
+    elif sys.platform == "emscripten":
+        match_str = r"\[Errno 44] .*'/foo"
     else:
         match_str = r"\[Errno 2] .*'/foo"
     with pytest.raises(FileNotFoundError, match=match_str):
diff --git a/lib/matplotlib/tests/test_dviread.py b/lib/matplotlib/tests/test_dviread.py
index 7b7ff151be18..2aa3abe12e4b 100644
--- a/lib/matplotlib/tests/test_dviread.py
+++ b/lib/matplotlib/tests/test_dviread.py
@@ -1,6 +1,7 @@
 import json
 from pathlib import Path
 import shutil
+import sys
 
 import matplotlib.dviread as dr
 import pytest
@@ -60,7 +61,7 @@ def test_PsfontsMap(monkeypatch):
         fontmap[b'%']
 
 
-@pytest.mark.skipif(shutil.which("kpsewhich") is None,
+@pytest.mark.skipif(sys.platform == "emscripten" or shutil.which("kpsewhich") is None,
                     reason="kpsewhich is not available")
 def test_dviread():
     dirpath = Path(__file__).parent / 'baseline_images/dviread'
diff --git a/lib/matplotlib/tests/test_figure.py b/lib/matplotlib/tests/test_figure.py
index c5890a2963b3..702b51ca590a 100644
--- a/lib/matplotlib/tests/test_figure.py
+++ b/lib/matplotlib/tests/test_figure.py
@@ -3,6 +3,7 @@
 import io
 import pickle
 import platform
+import sys
 from threading import Timer
 from types import SimpleNamespace
 import warnings
@@ -1605,6 +1606,8 @@ def test_add_axes_kwargs():
     plt.close()
 
 
+@pytest.mark.skipif(sys.platform == 'emscripten',
+                    reason='emscripten does not support threads')
 def test_ginput(recwarn):  # recwarn undoes warn filters at exit.
     warnings.filterwarnings("ignore", "cannot show the figure")
     fig, ax = plt.subplots()
@@ -1627,6 +1630,8 @@ def multi_presses():
     np.testing.assert_allclose(fig.ginput(3), [(.3, .4), (.5, .6)])
 
 
+@pytest.mark.skipif(sys.platform == 'emscripten',
+                    reason='emscripten does not support threads')
 def test_waitforbuttonpress(recwarn):  # recwarn undoes warn filters at exit.
     warnings.filterwarnings("ignore", "cannot show the figure")
     fig = plt.figure()
diff --git a/lib/matplotlib/tests/test_font_manager.py b/lib/matplotlib/tests/test_font_manager.py
index 2253596ac3e2..8a1537350ca7 100644
--- a/lib/matplotlib/tests/test_font_manager.py
+++ b/lib/matplotlib/tests/test_font_manager.py
@@ -228,6 +228,8 @@ def _model_handler(_):
     plt.close()
 
 
+@pytest.mark.skipif(sys.platform == 'emscripten',
+                    reason='emscripten does not support subprocesses')
 @pytest.mark.skipif(not hasattr(os, "register_at_fork"),
                     reason="Cannot register at_fork handlers")
 def test_fork():
diff --git a/lib/matplotlib/texmanager.py b/lib/matplotlib/texmanager.py
index 020a26e31cbe..993f9b0aeb6e 100644
--- a/lib/matplotlib/texmanager.py
+++ b/lib/matplotlib/texmanager.py
@@ -255,10 +255,6 @@ def _run_checked_subprocess(cls, command, tex, *, cwd=None):
             report = subprocess.check_output(
                 command, cwd=cwd if cwd is not None else cls._texcache,
                 stderr=subprocess.STDOUT)
-        except FileNotFoundError as exc:
-            raise RuntimeError(
-                f'Failed to process string with tex because {command[0]} '
-                'could not be found') from exc
         except subprocess.CalledProcessError as exc:
             raise RuntimeError(
                 '{prog} was not able to process the following string:\n'
@@ -271,6 +267,10 @@ def _run_checked_subprocess(cls, command, tex, *, cwd=None):
                     tex=tex.encode('unicode_escape'),
                     exc=exc.output.decode('utf-8', 'backslashreplace'))
                 ) from None
+        except (FileNotFoundError, OSError) as exc:
+            raise RuntimeError(
+                f'Failed to process string with tex because {command[0]} '
+                'could not be found') from exc
         _log.debug(report)
         return report
 

From bdbeb32122f13e8ef93cfc7f3b0e1c6c74431e3a Mon Sep 17 00:00:00 2001
From: Elliott Sales de Andrade <quantum.analyst@gmail.com>
Date: Wed, 6 Nov 2024 05:20:05 -0500
Subject: [PATCH 07/11] Skip tests that expect LinAlgError on WASM

---
 lib/matplotlib/tests/test_mlab.py | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/lib/matplotlib/tests/test_mlab.py b/lib/matplotlib/tests/test_mlab.py
index 109a6d542450..82f877b4cc01 100644
--- a/lib/matplotlib/tests/test_mlab.py
+++ b/lib/matplotlib/tests/test_mlab.py
@@ -884,6 +884,8 @@ def test_single_dataset_element(self):
         with pytest.raises(ValueError):
             mlab.GaussianKDE([42])
 
+    @pytest.mark.skipif(sys.platform == 'emscripten',
+                        reason="WASM doesn't support floating-point exceptions")
     def test_silverman_multidim_dataset(self):
         """Test silverman's for a multi-dimensional array."""
         x1 = np.array([[1, 2, 3], [4, 5, 6], [7, 8, 9]])
@@ -897,6 +899,8 @@ def test_silverman_singledim_dataset(self):
         y_expected = 0.76770389927475502
         assert_almost_equal(mygauss.covariance_factor(), y_expected, 7)
 
+    @pytest.mark.skipif(sys.platform == 'emscripten',
+                        reason="WASM doesn't support floating-point exceptions")
     def test_scott_multidim_dataset(self):
         """Test scott's output for a multi-dimensional array."""
         x1 = np.array([[1, 2, 3], [4, 5, 6], [7, 8, 9]])

From 3a1a61e3b374339d3039c4193ff06b208a3faf14 Mon Sep 17 00:00:00 2001
From: Elliott Sales de Andrade <quantum.analyst@gmail.com>
Date: Wed, 6 Nov 2024 00:25:44 -0500
Subject: [PATCH 08/11] ci: Add a build for wasm

This adds a `pyproject.toml` config for it, so you can replicate locally
with cibuildwheel.
---
 .github/labeler.yml             |  4 ++-
 .github/workflows/nightlies.yml | 40 +++++++++++------------
 .github/workflows/wasm.yml      | 58 +++++++++++++++++++++++++++++++++
 pyproject.toml                  | 20 ++++++++++++
 4 files changed, 101 insertions(+), 21 deletions(-)
 create mode 100644 .github/workflows/wasm.yml

diff --git a/.github/labeler.yml b/.github/labeler.yml
index 75adfed57f43..77b79146b47f 100644
--- a/.github/labeler.yml
+++ b/.github/labeler.yml
@@ -1,7 +1,9 @@
 ---
 "CI: Run cibuildwheel":
   - changed-files:
-      - any-glob-to-any-file: ['.github/workflows/cibuildwheel.yml']
+      - any-glob-to-any-file:
+          - '.github/workflows/cibuildwheel.yml'
+          - '.github/workflows/wasm.yml'
 "CI: Run cygwin":
   - changed-files:
       - any-glob-to-any-file: ['.github/workflows/cygwin.yml']
diff --git a/.github/workflows/nightlies.yml b/.github/workflows/nightlies.yml
index 393ce2e73472..b534aa3ef5b0 100644
--- a/.github/workflows/nightlies.yml
+++ b/.github/workflows/nightlies.yml
@@ -31,28 +31,28 @@ jobs:
         run: |
           PROJECT_REPO="matplotlib/matplotlib"
           BRANCH="main"
-          WORKFLOW_NAME="cibuildwheel.yml"
           ARTIFACT_PATTERN="cibw-wheels-*"
 
-          gh run --repo "${PROJECT_REPO}" \
-             list --branch "${BRANCH}" \
-                  --workflow "${WORKFLOW_NAME}" \
-                  --json event,status,conclusion,databaseId > runs.json
-          RUN_ID=$(
-            jq --compact-output \
-              '[
-                .[] |
-                # Filter on "push" events to main (merged PRs) ...
-                select(.event == "push") |
-                # that have completed successfully ...
-                select(.status == "completed" and .conclusion == "success")
-               ] |
-              # and get ID of latest build of wheels.
-              sort_by(.databaseId) | reverse | .[0].databaseId' runs.json
-          )
-          gh run --repo "${PROJECT_REPO}" view "${RUN_ID}"
-          gh run --repo "${PROJECT_REPO}" \
-             download "${RUN_ID}" --pattern "${ARTIFACT_PATTERN}"
+          for WORKFLOW_NAME in cibuildwheel.yml wasm.yml; do
+            gh run --repo "${PROJECT_REPO}" \
+               list --branch "${BRANCH}" \
+                    --workflow "${WORKFLOW_NAME}" \
+                    --json event,status,conclusion,databaseId > runs.json
+            RUN_ID=$(
+              jq --compact-output \
+                '[
+                  .[] |
+                  # Filter on "push" events to main (merged PRs) ...
+                  select(.event == "push") |
+                  # that have completed successfully ...
+                  select(.status == "completed" and .conclusion == "success")
+                 ] |
+                # and get ID of latest build of wheels.
+                sort_by(.databaseId) | reverse | .[0].databaseId' runs.json
+            )
+            gh run --repo "${PROJECT_REPO}" view "${RUN_ID}"
+            gh run --repo "${PROJECT_REPO}" download "${RUN_ID}" --pattern "${ARTIFACT_PATTERN}"
+          done
 
           mkdir dist
           mv ${ARTIFACT_PATTERN}/*.whl dist/
diff --git a/.github/workflows/wasm.yml b/.github/workflows/wasm.yml
new file mode 100644
index 000000000000..38872d6439d0
--- /dev/null
+++ b/.github/workflows/wasm.yml
@@ -0,0 +1,58 @@
+---
+name: Build wasm wheels
+
+on:
+  # Save CI by only running this on release branches or tags.
+  push:
+    branches:
+      - main
+      - v[0-9]+.[0-9]+.x
+    tags:
+      - v*
+  # Also allow running this action on PRs if requested by applying the
+  # "Run cibuildwheel" label.
+  pull_request:
+    types:
+      - opened
+      - synchronize
+      - reopened
+      - labeled
+
+permissions:
+  contents: read
+
+jobs:
+  build_wasm:
+    if: >-
+      github.event_name == 'push' ||
+      github.event_name == 'pull_request' && (
+        (
+          github.event.action == 'labeled' &&
+          github.event.label.name == 'CI: Run cibuildwheel'
+        ) ||
+        contains(github.event.pull_request.labels.*.name,
+                 'CI: Run cibuildwheel')
+      )
+    name: Build wasm
+    runs-on: ubuntu-24.04
+
+    steps:
+      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683  # v4.2.2
+        with:
+          fetch-depth: 0
+
+      - uses: actions/setup-python@a26af69be951a213d495a4c3e4e4022e16d87065  # v5.6.0
+        name: Install Python
+        with:
+          python-version: '3.13'
+
+      - name: Build wheels for wasm
+        uses: pypa/cibuildwheel@faf86a6ed7efa889faf6996aa23820831055001a  # v2.23.3
+        env:
+          CIBW_PLATFORM: "pyodide"
+
+      - uses: actions/upload-artifact@ea165f8d65b6e75b540449e92b4886f43607fa02  # v4.6.2
+        with:
+          name: cibw-wheels-wasm
+          path: ./wheelhouse/*.whl
+          if-no-files-found: error
diff --git a/pyproject.toml b/pyproject.toml
index b580feff930e..64393eb51358 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -319,3 +319,23 @@ testpaths = ["lib"]
 addopts = [
     "--import-mode=importlib",
 ]
+
+[tool.cibuildwheel.pyodide]
+test-requires = "pytest"
+test-command = [
+    # Wheels are built without test images, so copy them into the testing directory.
+    "basedir=$(python -c 'import pathlib, matplotlib; print(pathlib.Path(matplotlib.__file__).parent.parent)')",
+    "cp -a {package}/lib/matplotlib/tests/data $basedir/matplotlib/tests/",
+    """
+    for subdir in matplotlib mpl_toolkits/axes_grid1 mpl_toolkits/axisartist mpl_toolkits/mplot3d; do
+        cp -a {package}/lib/${subdir}/tests/baseline_images $basedir/${subdir}/tests/
+    done""",
+    # Test installed, not repository, copy as we aren't using an editable install.
+    "pytest -p no:cacheprovider --pyargs matplotlib mpl_toolkits.axes_grid1 mpl_toolkits.axisartist mpl_toolkits.mplot3d",
+]
+[tool.cibuildwheel.pyodide.environment]
+# Exceptions are needed for pybind11:
+# https://github.com/pybind/pybind11/pull/5298
+CFLAGS = "-fexceptions"
+CXXFLAGS = "-fexceptions"
+LDFLAGS = "-fexceptions"

From 7b48eef9f6c987c21c57c7dd1bd91637f80cc2d3 Mon Sep 17 00:00:00 2001
From: Elliott Sales de Andrade <quantum.analyst@gmail.com>
Date: Wed, 6 Nov 2024 01:20:52 -0500
Subject: [PATCH 09/11] Open symbol visibility for FreeType on wasm

---
 subprojects/packagefiles/freetype-2.6.1-meson/meson.build | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/subprojects/packagefiles/freetype-2.6.1-meson/meson.build b/subprojects/packagefiles/freetype-2.6.1-meson/meson.build
index 9a5180ef7586..1fd4bc44e7b5 100644
--- a/subprojects/packagefiles/freetype-2.6.1-meson/meson.build
+++ b/subprojects/packagefiles/freetype-2.6.1-meson/meson.build
@@ -179,11 +179,17 @@ ft_config_headers += [configure_file(input: 'include/freetype/config/ftoption.h.
   output: 'ftoption.h',
   configuration: conf)]
 
+if cc.get_id() == 'emscripten'
+  kwargs = {}
+else
+  kwargs = {'gnu_symbol_visibility': 'inlineshidden'}
+endif
+
 libfreetype = static_library('freetype', base_sources,
   include_directories: incbase,
   dependencies: deps,
   c_args: c_args,
-  gnu_symbol_visibility: 'inlineshidden',
+  kwargs: kwargs
 )
 
 freetype_dep = declare_dependency(

From e35fc24185a27fa90fb6af7b3c0579443daf8c48 Mon Sep 17 00:00:00 2001
From: Elliott Sales de Andrade <quantum.analyst@gmail.com>
Date: Thu, 7 Nov 2024 18:53:55 -0500
Subject: [PATCH 10/11] TST: Avoid using os.devnull for path tests

On wasm, this file doesn't support seeking, which is sometimes necessary
depending on file type.
---
 lib/matplotlib/tests/test_backend_pdf.py | 5 ++---
 lib/matplotlib/tests/test_image.py       | 4 ++--
 2 files changed, 4 insertions(+), 5 deletions(-)

diff --git a/lib/matplotlib/tests/test_backend_pdf.py b/lib/matplotlib/tests/test_backend_pdf.py
index f126fb543e78..a3e9895d8e4d 100644
--- a/lib/matplotlib/tests/test_backend_pdf.py
+++ b/lib/matplotlib/tests/test_backend_pdf.py
@@ -1,7 +1,6 @@
 import datetime
 import decimal
 import io
-import os
 from pathlib import Path
 
 import numpy as np
@@ -291,8 +290,8 @@ def test_text_urls_tex():
             assert annot.Rect[1] == decimal.Decimal('0.7') * 72
 
 
-def test_pdfpages_fspath():
-    with PdfPages(Path(os.devnull)) as pdf:
+def test_pdfpages_fspath(tmp_path):
+    with PdfPages(tmp_path / 'unused.pdf') as pdf:
         pdf.savefig(plt.figure())
 
 
diff --git a/lib/matplotlib/tests/test_image.py b/lib/matplotlib/tests/test_image.py
index 00c223c59362..e3d425ba3460 100644
--- a/lib/matplotlib/tests/test_image.py
+++ b/lib/matplotlib/tests/test_image.py
@@ -215,8 +215,8 @@ def test_imsave_rgba_origin(origin):
 
 
 @pytest.mark.parametrize("fmt", ["png", "pdf", "ps", "eps", "svg"])
-def test_imsave_fspath(fmt):
-    plt.imsave(Path(os.devnull), np.array([[0, 1]]), format=fmt)
+def test_imsave_fspath(fmt, tmp_path):
+    plt.imsave(tmp_path / f'unused.{fmt}', np.array([[0, 1]]), format=fmt)
 
 
 def test_imsave_color_alpha():

From 5eb8a4249c8cfc821dd65430e1f35606df086cc1 Mon Sep 17 00:00:00 2001
From: Elliott Sales de Andrade <quantum.analyst@gmail.com>
Date: Fri, 16 May 2025 19:30:26 -0400
Subject: [PATCH 11/11] Increase some tolerances for wasm

---
 lib/matplotlib/tests/test_axes.py   | 4 ++--
 lib/matplotlib/tests/test_legend.py | 8 ++++----
 2 files changed, 6 insertions(+), 6 deletions(-)

diff --git a/lib/matplotlib/tests/test_axes.py b/lib/matplotlib/tests/test_axes.py
index c96173e340f7..495f773afa69 100644
--- a/lib/matplotlib/tests/test_axes.py
+++ b/lib/matplotlib/tests/test_axes.py
@@ -8277,7 +8277,7 @@ def test_normal_axes():
     ]
     for nn, b in enumerate(bbaxis):
         targetbb = mtransforms.Bbox.from_bounds(*target[nn])
-        assert_array_almost_equal(b.bounds, targetbb.bounds, decimal=2)
+        assert_array_almost_equal(b.bounds, targetbb.bounds, decimal=1)
 
     target = [
         [150.0, 119.999, 930.0, 11.111],
@@ -8295,7 +8295,7 @@ def test_normal_axes():
 
     target = [85.5138, 75.88888, 1021.11, 1017.11]
     targetbb = mtransforms.Bbox.from_bounds(*target)
-    assert_array_almost_equal(bbtb.bounds, targetbb.bounds, decimal=2)
+    assert_array_almost_equal(bbtb.bounds, targetbb.bounds, decimal=1)
 
     # test that get_position roundtrips to get_window_extent
     axbb = ax.get_position().transformed(fig.transFigure).bounds
diff --git a/lib/matplotlib/tests/test_legend.py b/lib/matplotlib/tests/test_legend.py
index 9b100037cc41..c18b52563042 100644
--- a/lib/matplotlib/tests/test_legend.py
+++ b/lib/matplotlib/tests/test_legend.py
@@ -513,10 +513,10 @@ def test_figure_legend_outside():
         leg = fig.legend(loc='outside ' + todo)
         fig.draw_without_rendering()
 
-        assert_allclose(axs.get_window_extent().extents,
-                        axbb[nn])
-        assert_allclose(leg.get_window_extent().extents,
-                        legbb[nn])
+        assert_allclose(axs.get_window_extent().extents, axbb[nn],
+                        rtol=1e-4)
+        assert_allclose(leg.get_window_extent().extents, legbb[nn],
+                        rtol=1e-4)
 
 
 @image_comparison(['legend_stackplot.png'],