[singlehtml]: append docname to section id

Author: gastmaier

AUTHORS.rst

@@ -72,6 +72,7 @@ Contributors
 * Joel Wurtz -- cellspanning support in LaTeX
 * John Waltman -- Texinfo builder
 * Jon Dufresne -- modernisation
+* Jorge Marques -- singlehtml unique section ids
 * Josip Dzolonga -- coverage builder
 * Juan Luis Cano Rodríguez -- new tutorial (2021)
 * Julien Palard -- Colspan and rowspan in text builder

CHANGES.rst

@@ -116,6 +116,9 @@ Bugs fixed
 * #10785: Autodoc: Allow type aliases defined in the project to be properly
   cross-referenced when used as type annotations. This makes it possible
   for objects documented as ``:py:data:`` to be hyperlinked in function signatures.
+* #13738: singlehtml builder: make section ids unique by appending the docname,
+  matching ``sphinx/environment/adapters/toctree.py``'s ``_resolve_toctree()``
+  format. E.g., ``id3`` becomes ``document-path/to/doc#id3``.
 
 Testing
 -------

sphinx/writers/html5.py

@@ -17,7 +17,7 @@
 from sphinx.util.images import get_image_size
 
 if TYPE_CHECKING:
-    from docutils.nodes import Element, Node, Text
+    from docutils.nodes import Element, Node, Text, section
 
     from sphinx.builders import Builder
     from sphinx.builders.html import StandaloneHTMLBuilder
@@ -497,6 +497,15 @@ def depart_term(self, node: Element) -> None:
 
             self.body.append('</dt>')
 
+    def visit_section(self, node: section) -> None:
+        if self.builder.name == 'singlehtml' and node['ids']:
+            docname = self.docnames[-1]
+            node['ids'][0] = 'document-' + docname + '#' + node['ids'][0]
+        super().visit_section(node)
+
+    def depart_section(self, node: section) -> None:
+        super().depart_section(node)
+
     # overwritten
     def visit_title(self, node: nodes.title) -> None:
         if (