fix: allow indented lines in setext heading text

[giaBaoJS]

Marked version: 18.0.13 (634f810)

Markdown flavor: CommonMark

Description

The setext heading rule refuses to let the heading text span a line that looks like an indented code block, and it lets a thematic break line be swallowed into the heading text. Both come from the same list of interrupters in lheadingCore.

Expectation

CommonMark 0.31.2, section 4.3 Setext headings:

A setext heading consists of one or more lines of text, not interrupted by a blank line, of which the first line does not have more than 3 spaces of indentation, followed by a setext heading underline. The lines of text must be such that, were they not followed by the setext heading underline, they would be interpreted as a paragraph: they cannot be interpretable as a code fence, ATX heading, block quote, thematic break, list item, or HTML block.

Indented code is deliberately absent from that list, because (4.4 Indented code blocks) an indented code block cannot interrupt a paragraph, and (4.8 Paragraphs) "lines after the first may be indented any amount, since indented code blocks cannot interrupt paragraphs". Thematic break is on the list, but was missing from the rule.

src/Lexer.ts already says the same thing in words for paragraphs (// An indented code block cannot interrupt a paragraph.), and _paragraph correctly omits indented code from its own continuation interrupters.

Result

commonmark@0.31.2 (already a devDependency here) is the reference:

input	marked 18.0.13	commonmark 0.31.2
a\n b\n===	<p>a\n b\n===</p>	<h1>a\nb</h1>
a\n b\n---	<p>a\n b</p>\n<hr>	<h2>a\nb</h2>
a\n\tb\n===	<p>a\n\tb\n===</p>	<h1>a\nb</h1>
a\n***\n===	<h1>a\n***</h1>	<p>a</p>\n<hr>\n<p>===</p>

What was attempted

src/rules.ts:115 uses one placeholder list in two positions:

const lheadingCore = /^(?!bull |blockCode|fences|...)((?:.|\n(?!\s*?\n|bull |blockCode|fences|...))+?)\n {0,3}(=+|-+) *(?:\n+|$)/;

The leading (?!...) guard is correct and stays: at the start of a block an indented line really does open a code block ( Foo\n=== is a code block plus a paragraph, and marked already gets that right). Only the continuation lookahead is wrong, so blockCode is dropped from it and hr is added, leaving the two positions with the lists the spec describes.

The hr half is not optional. Without it, allowing indented continuation lines exposes the pre-existing gap for thematic breaks on many more inputs, and the change is a net regression; with it, the differential fuzz below is clean.

Measurements

Suites, before and after (node --test):

suite	before	after
test:specs	1819 pass / 0 fail / 0 skip	1823 pass / 0 fail / 0 skip
test:unit	191 pass / 0 fail / 0 skip	191 pass / 0 fail / 0 skip

The 4 added spec tests are the two new fixture pairs (gfm and gfm: false).

Conformance, measured by running every example in test/specs/commonmark and test/specs/gfm through marked and comparing with the same html-differ the harness uses, rather than reading the completion table (which counts shouldFail annotations and is static):

suite	before	after
CommonMark 0.31.2 (gfm: false)	649/652 = 99.54%	649/652 = 99.54%
CommonMark 0.31.2 (gfm: true)	645/652 = 98.93%	645/652 = 98.93%
GFM extensions	24/28 = 85.71%	24/28 = 85.71%

Unchanged: no example in the CommonMark suite covers a setext heading with an indented continuation line, which is why this went unnoticed. The same failing examples fail before and after (32, 33, 503, and 602/608/611/612 under gfm).

Differential fuzz over 220,000 generated block-structure documents (random line sequences drawn from indented lines, tabs, underlines, fences, bullets, quotes, ATX headings, thematic breaks, table rows, html tags), comparing old marked / new marked / commonmark@0.31.2 with the harness's html-differ, across 5 seeds and both gfm: false and gfm: true:

seed A gfm:false  60000 docs  1173 fixed  0 regressions
seed B gfm:false  40000 docs   839 fixed  0 regressions
seed C gfm:false  40000 docs   791 fixed  0 regressions
seed D gfm:true   40000 docs   694 fixed  0 regressions
seed E gfm:true   40000 docs   698 fixed  0 regressions

The handful of remaining documents where both versions differ from the reference differ only on unrelated pre-existing points (link reference definitions, <br> formatting).

Two things worth flagging:

• marked keeps the continuation line's leading indentation inside the heading text (<h1>a\n b</h1> where the reference emits <h1>a\nb</h1>). That is pre-existing and identical to what marked already does for paragraph continuation lines (<p>a\n b</p>), it is whitespace-insignificant in the rendered HTML, and the spec suite's differ treats the two as equal. The fixtures are therefore written without renderExact. Happy to fix the whitespace separately if you want it.
• blockPedantic.paragraph embeds lheading in its continuation lookahead, so pedantic mode inherits the change. test/specs/original (pedantic) stays 100% green.

recheck reports block.normal.lheading, block.gfm.lheading and block.pedantic.lheading as safe after the change.

Contributor

• Test(s) exist to ensure functionality and minimize regression (if no tests added, list tests covering this PR); or,
• no tests required for this PR.
• If submitting new feature, it has been documented in the appropriate places.

Committer

In most cases, this should be a different person than the contributor.

• CI is green (no forced merge required).
• Squash and Merge PR following conventional commit guidelines.

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[UziTech]

Thanks! 💯