Ensure code blocks (<pre>) are keyboard focusable (#1636)

* Ensure code blocks (<pre>) are keyboard focusable

* Also cover <pre> tags not covered by starttag

* Update src/pydata_sphinx_theme/translator.py

Co-authored-by: gabalafou <[email protected]>

---------

Co-authored-by: Tania Allard <[email protected]>

Author: gabalafou

src/pydata_sphinx_theme/translator.py

@@ -3,6 +3,7 @@
 import types
 
 import sphinx
+from docutils import nodes
 from packaging.version import Version
 from sphinx.application import Sphinx
 from sphinx.ext.autosummary import autosummary_table
@@ -23,11 +24,35 @@ def __init__(self, *args, **kwds):
         self.settings.table_style = "table"
 
     def starttag(self, *args, **kwargs):
-        """Ensure an aria-level is set for any heading role."""
+        """Perform small modifications to tags.
+
+        - ensure aria-level is set for any tag with heading role
+        - ensure <pre> tags have tabindex="0".
+        """
         if kwargs.get("ROLE") == "heading" and "ARIA-LEVEL" not in kwargs:
             kwargs["ARIA-LEVEL"] = "2"
+
+        if "pre" in args:
+            kwargs["tabindex"] = "0"
+
         return super().starttag(*args, **kwargs)
 
+    def visit_literal_block(self, node):
+        """Modify literal blocks.
+
+        - add tabindex="0" to <pre> tags within the HTML tree of the literal
+          block
+        """
+        try:
+            super().visit_literal_block(node)
+        except nodes.SkipNode:
+            # If the super method raises nodes.SkipNode, then we know it
+            # executed successfully and appended to self.body a string of HTML
+            # representing the code block, which we then modify.
+            html_string = self.body[-1]
+            self.body[-1] = html_string.replace("<pre", '<pre tabindex="0"')
+            raise nodes.SkipNode
+
     def visit_table(self, node):
         """Custom visit table method.