Closes #13022: fix _toctree_copy with multi level 'only' nodes.

When copying toc tree, 'only' nodes should not be copied, but previous
implementation always considered 'only' node children as duplicable nodes which
is wrong when 'only' nodes is parent of other 'only' nodes.

sphinx/environment/adapters/toctree.py

@@ -464,16 +464,13 @@ def _toctree_add_classes(node: Element, depth: int, docname: str) -> None:
 
 ET = TypeVar('ET', bound=Element)
 
-
-def _toctree_copy(
-    node: ET, depth: int, maxdepth: int, collapse: bool, tags: Tags
-) -> ET:
+def _toctree_copy_children(
+    copy: ET, node: ET, depth: int, maxdepth: int, collapse: bool, tags: Tags
+) -> None:
     """Utility: Cut and deep-copy a TOC at a specified depth."""
     keep_bullet_list_sub_nodes = depth <= 1 or (
         (depth <= maxdepth or maxdepth <= 0) and (not collapse or 'iscurrent' in node)
     )
-
-    copy = node.copy()
     for subnode in node.children:
         if isinstance(subnode, addnodes.compact_paragraph | nodes.list_item):
             # for <p> and <li>, just recurse
@@ -489,18 +486,11 @@ def _toctree_copy(
             # copy sub toctree nodes for later processing
             copy.append(subnode.copy())
         elif isinstance(subnode, addnodes.only):
-            # only keep children if the only node matches the tags
+            # Only keep its children if the 'only' node matches the tags. When
+            # matching, call recursively the current function without
+            # incrementing the depth as 'only' nodes are not toctree nodes.
             if _only_node_keep_children(subnode, tags):
-                for child in subnode.children:
-                    copy.append(
-                        _toctree_copy(
-                            child,
-                            depth,
-                            maxdepth,
-                            collapse,
-                            tags,  # type: ignore[type-var]
-                        )
-                    )
+                _toctree_copy_children(copy, subnode, depth, maxdepth, collapse, tags)
         elif isinstance(subnode, nodes.reference | nodes.title):
             # deep copy references and captions
             sub_node_copy = subnode.copy()
@@ -511,6 +501,15 @@ def _toctree_copy(
         else:
             msg = f'Unexpected node type {subnode.__class__.__name__!r}!'
             raise ValueError(msg)
+
+    return copy
+
+
+def _toctree_copy(
+    node: ET, depth: int, maxdepth: int, collapse: bool, tags: Tags
+) -> ET:
+    copy = node.copy()
+    _toctree_copy_children(copy, node, depth, maxdepth, collapse, tags)
     return copy