Ensure lines after heading in loose list are properly detabbed

This is a weird edge case. Normally, detabbing would be handled by
the `ListIndentProcessor`. However, in this one case, that class's
`get_level` method would need to return a different result than in
any other case. As there is no way to easily determine this specific
case from that class, we make the adjustment directly in the
`HashHeaderProcessor` class. Fixes #1433.

Author: waylan

Diff for: .spell-dict

@@ -32,6 +32,7 @@ customizable
 dedent
 deliminators
 deregister
+detabbed
 Dmitry
 docdata
 ElementTree

Diff for: docs/changelog.md

@@ -13,6 +13,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Fixed
 
 * Include `scripts/*.py` in the generated source tarballs (#1430).
+* Ensure lines after heading in loose list are properly detabbed (#1443).
 
 ## [3.5.2] -- 2024-01-10
 

Diff for: markdown/blockprocessors.py

@@ -479,6 +479,11 @@ def run(self, parent: etree.Element, blocks: list[str]) -> None:
             h.text = m.group('header').strip()
             if after:
                 # Insert remaining lines as first block for future parsing.
+                if self.parser.state.isstate('looselist'):
+                    # This is a weird edge case where a header is a child of a loose list
+                    # and there is no blank line after the header. To ensure proper
+                    # parsing, the line(s) after need to be detabbed. See #1443.
+                    after = self.looseDetab(after)
                 blocks.insert(0, after)
         else:  # pragma: no cover
             # This should never happen, but just in case...

Diff for: tests/test_syntax/blocks/test_ul.py

@@ -0,0 +1,137 @@
+"""
+Python Markdown
+
+A Python implementation of John Gruber's Markdown.
+
+Documentation: https://python-markdown.github.io/
+GitHub: https://github.com/Python-Markdown/markdown/
+PyPI: https://pypi.org/project/Markdown/
+
+Started by Manfred Stienstra (http://www.dwerg.net/).
+Maintained for a few years by Yuri Takhteyev (http://www.freewisdom.org).
+Currently maintained by Waylan Limberg (https://github.com/waylan),
+Dmitry Shachnev (https://github.com/mitya57) and Isaac Muse (https://github.com/facelessuser).
+
+Copyright 2007-2024 The Python Markdown Project (v. 1.7 and later)
+Copyright 2004, 2005, 2006 Yuri Takhteyev (v. 0.2-1.6b)
+Copyright 2004 Manfred Stienstra (the original version)
+
+License: BSD (see LICENSE.md for details).
+"""
+
+import unittest
+from markdown.test_tools import TestCase
+
+
+class TestUnorderedLists(TestCase):
+
+    # TODO: Move legacy tests here
+
+    def test_header_and_paragraph_no_blank_line_loose_list(self):
+        self.assertMarkdownRenders(
+            self.dedent(
+                """
+                - ### List 1
+                    Entry 1.1
+
+                - ### List 2
+                    Entry 2.1
+                """
+            ),
+            self.dedent(
+                """
+                <ul>
+                <li>
+                <h3>List 1</h3>
+                <p>Entry 1.1</p>
+                </li>
+                <li>
+                <h3>List 2</h3>
+                <p>Entry 2.1</p>
+                </li>
+                </ul>
+                """
+            )
+        )
+
+    # Note: This is strange. Any other element on first line of list item would get very
+    # different behavior. However, as a heading can only ever be one line, this is the
+    # correct behavior. In fact, `markdown.pl` behaves this way with no indentation.
+    def test_header_and_paragraph_no_blank_line_loose_list_no_indent(self):
+        self.assertMarkdownRenders(
+            self.dedent(
+                """
+                - ### List 1
+                Entry 1.1
+
+                - ### List 2
+                Entry 2.1
+                """
+            ),
+            self.dedent(
+                """
+                <ul>
+                <li>
+                <h3>List 1</h3>
+                <p>Entry 1.1</p>
+                </li>
+                <li>
+                <h3>List 2</h3>
+                <p>Entry 2.1</p>
+                </li>
+                </ul>
+                """
+            )
+        )
+
+    # TODO: Possibly change this behavior. While this test follows the behavior
+    # of `markdown.pl`, it is likely surprising to most users. In fact, actual
+    # behavior is to return the same results as for a loose list.
+    @unittest.skip('This behaves as a loose list in Python-Markdown')
+    def test_header_and_paragraph_no_blank_line_tight_list(self):
+        self.assertMarkdownRenders(
+            self.dedent(
+                """
+                - ### List 1
+                    Entry 1.1
+                - ### List 2
+                    Entry 2.1
+                """
+            ),
+            self.dedent(
+                """
+                <ul>
+                <li>### List 1
+                Entry 1.1</li>
+                <li>### List 2
+                Entry 2.1</li>
+                </ul>
+                """
+            )
+        )
+
+    # TODO: Possibly change this behavior. While this test follows the behavior
+    # of `markdown.pl`, it is likely surprising to most users. In fact, actual
+    # behavior is to return the same results as for a loose list.
+    @unittest.skip('This behaves as a loose list in Python-Markdown')
+    def test_header_and_paragraph_no_blank_line_tight_list_no_indent(self):
+        self.assertMarkdownRenders(
+            self.dedent(
+                """
+                - ### List 1
+                Entry 1.1
+                - ### List 2
+                Entry 2.1
+                """
+            ),
+            self.dedent(
+                """
+                <ul>
+                <li>### List 1
+                Entry 1.1</li>
+                <li>### List 2
+                Entry 2.1</li>
+                </ul>
+                """
+            )
+        )