🐛 HTML block start outside a list item closes the list

[inchang-ing]

Fixes #434

Problem

A line that starts an HTML block (any type) on a line under-indented for the list item's content was consumed as a lazy continuation of the item's paragraph:

## Opts

- a
<br>
## Next

rendered as <li>a\n<br></li> in markdown-it-py (and in the JS original), while cmark and commonmark.js close the list and open the HTML block at the outer level — because such a line can neither be item content nor a lazy continuation of it (it starts a new block). Flipping the type-7 "can terminate paragraph" flag is not a fix: it would let <br> interrupt a top-level paragraph, where all implementations agree it must not.

Change

• html_block.py: new html_block_starts(state, line) helper — detects whether a line begins an HTML block of any type (same sequence table as the rule).
• paragraph.py: while scanning continuation lines, end the paragraph when the next line starts an HTML block and is under-indented for the current container (sCount < blkIndent). The list rule then sees the line, finds no item marker, and closes the list, so the line is offered to the outer containers — reproducing cmark's container walk. At the same container level a type-7 sequence still does not interrupt the paragraph (the terminator rules are unchanged), and plain text at outer indentation remains a lazy continuation.
• CHANGELOG.md: entry under Unreleased, per the contributing guidance in AGENTS.md.

Verification

• The issue's minimal input now renders exactly the cmark output: <h2>Opts</h2><ul><li>a</li></ul><br>\n## Next (the type-7 block swallows ## Next up to a blank line, as cmark does).
• Five cases are pinned in tests/test_html_block_list.py: the issue's input; HTML block at the item's content indent stays a lazy continuation; a top-level paragraph still continues through <br>; plain text at outer indentation is still a lazy continuation; a type-6 block (<div>) still closes the list as before.
• Full suite with CI's dependency set (.[testing,linkify]): 1037 passed, 0 failed; pristine master in the same environment: 1032 passed — the delta is exactly the 5 new tests, so zero regressions (CommonMark spec tests included).
• Reverting the two source files while keeping the tests makes test_html_block_under_indented_closes_list fail, so the tests do pin the fix.
• ruff check / ruff format clean on touched files; mypy --strict clean (the tests build MarkdownIt locally instead of via a fixture, matching the example pattern in AGENTS.md — the pre-commit mypy environment does not have pytest installed, and an untyped decorator is not allowed under the repo's strict settings).

Note

The JS markdown-it behaves the same way as this library currently does, so this change intentionally diverges from upstream JS toward the cmark/commonmark.js behavior documented in the issue; happy to align on whatever the maintainers decide.

Disclosure

This PR was prepared with AI assistance (ZCode/GLM, orchestrated via WorkBuddy).

[inchang-ing]

Hi maintainers, gentle ping on #442 — fix(block): HTML block start outside a list item closes the list. It's a small, self-contained fix with passing tests. Would appreciate a review when you have a moment. Thanks for maintaining executablebooks/markdown-it-py!

[inchang-ing]

Addressed the pre-commit.ci mypy failure and tidied the PR up:

• mypy fix: tests/test_html_block_list.py used a @pytest.fixture, which the pre-commit mypy hook rejects — that hook's environment installs only mdurl/typing-extensions (no pytest), and the repo's strict mypy settings keep disallow_untyped_decorators enabled for tests.*, so Untyped decorator makes function "md" untyped. The tests now build MarkdownIt locally, matching the example test pattern in AGENTS.md and the rest of the suite (no other test file uses a fixture).
• CHANGELOG.md: added an entry under Unreleased, per the pull request requirements in AGENTS.md.
• Squashed the branch into a single commit on top of current master (the previous head also contained a pre-commit.ci autofix commit), and retitled to the EMOJI + description format the repo uses.

Verified locally with the same dependency set as CI (.[testing,linkify]): 1037 passed / 0 failed (pristine master: 1032 passed, i.e. the delta is exactly the 5 new tests), ruff check + ruff format clean, mypy --strict clean on the touched files.