Use Mathjax v4 by default (#14080)

Co-authored-by: Adam Turner <9087854+aa-turner@users.noreply.github.com>

tests/test_extensions/test_ext_math.py

@@ -17,6 +17,8 @@ from sphinx.testing.util import assert_node
 if TYPE_CHECKING:
     from sphinx.testing.util import SphinxTestApp
 
+FAKE_MATHJAX_URL = 'https://example.org/fake-mathjax.js'
+
 
 def has_binary(binary: str) -> bool:
     try:
@@ -125,7 +127,7 @@ def test_mathjax_options(app: SphinxTestApp) -> None:
     shutil.rmtree(app.outdir)
     assert (
         '<script defer="defer" integrity="sha384-0123456789" '
-        'src="https://cdn.jsdelivr.net/npm/mathjax@3/es5/tex-mml-chtml.js">'
+        f'src="{MATHJAX_URL}">'
         '</script>'
     ) in content
 
@@ -363,20 +365,38 @@ def test_math_compat(app: SphinxTestApp) -> None:
         )
 
 
+@pytest.mark.sphinx(
+    'html',
+    testroot='ext-math',
+    confoverrides={
+        'extensions': ['sphinx.ext.mathjax'],
+        'mathjax4_config': {'extensions': ['tex2jax.js']},
+    },
+)
+def test_mathjax4_config(app: SphinxTestApp) -> None:
+    app.build(force_all=True)
+
+    content = (app.outdir / 'index.html').read_text(encoding='utf8')
+    assert MATHJAX_URL in content
+    assert '<script defer="defer" src="%s">' % MATHJAX_URL in content
+    assert '<script>window.MathJax = {"extensions": ["tex2jax.js"]}</script>' in content
+
+
 @pytest.mark.sphinx(
     'html',
     testroot='ext-math',
     confoverrides={
         'extensions': ['sphinx.ext.mathjax'],
         'mathjax3_config': {'extensions': ['tex2jax.js']},
+        'mathjax_path': FAKE_MATHJAX_URL,
     },
 )
 def test_mathjax3_config(app: SphinxTestApp) -> None:
     app.build(force_all=True)
 
     content = (app.outdir / 'index.html').read_text(encoding='utf8')
-    assert MATHJAX_URL in content
-    assert '<script defer="defer" src="%s">' % MATHJAX_URL in content
+    assert FAKE_MATHJAX_URL in content
+    assert f'<script defer="defer" src="{FAKE_MATHJAX_URL}">' in content
     assert '<script>window.MathJax = {"extensions": ["tex2jax.js"]}</script>' in content
 
 
@@ -405,13 +425,14 @@ def test_mathjax_config_path_config(app: SphinxTestApp) -> None:
     confoverrides={
         'extensions': ['sphinx.ext.mathjax'],
         'mathjax2_config': {'extensions': ['tex2jax.js']},
+        'mathjax_path': FAKE_MATHJAX_URL,
     },
 )
 def test_mathjax2_config(app: SphinxTestApp) -> None:
     app.build(force_all=True)
 
     content = (app.outdir / 'index.html').read_text(encoding='utf8')
-    assert '<script async="async" src="%s">' % MATHJAX_URL in content
+    assert f'<script async="async" src="{FAKE_MATHJAX_URL}">' in content
     assert (
         '<script type="text/x-mathjax-config">'
         'MathJax.Hub.Config({"extensions": ["tex2jax.js"]})'
@@ -425,10 +446,10 @@ def test_mathjax2_config(app: SphinxTestApp) -> None:
     confoverrides={
         'extensions': ['sphinx.ext.mathjax'],
         'mathjax_options': {'async': 'async'},
-        'mathjax3_config': {'extensions': ['tex2jax.js']},
+        'mathjax4_config': {'extensions': ['tex2jax.js']},
     },
 )
-def test_mathjax_options_async_for_mathjax3(app: SphinxTestApp) -> None:
+def test_mathjax_options_async_for_mathjax4(app: SphinxTestApp) -> None:
     app.build(force_all=True)
 
     content = (app.outdir / 'index.html').read_text(encoding='utf8')
@@ -509,6 +530,7 @@ def test_mathjax_is_not_installed_if_no_equations(app: SphinxTestApp) -> None:
 
     content = (app.outdir / 'index.html').read_text(encoding='utf8')
     assert 'MathJax.js' not in content
+    assert MATHJAX_URL not in content
 
 
 @pytest.mark.sphinx(
@@ -575,6 +597,7 @@ def test_mathjax_is_not_installed_if_no_equations_singlehtml(
 
     content = (app.outdir / 'index.html').read_text(encoding='utf8')
     assert 'MathJax.js' not in content
+    assert MATHJAX_URL not in content
 
 
 @pytest.mark.sphinx(