fix: recognize task list items when a tab follows the checkbox - #4163

Marked version: 18.1.1 (also current master 4b7b5b2)

Markdown flavor: GitHub Flavored Markdown

Description

We did not find an existing issue or pull request for this. With gfm: true, a task marker followed by a tab is not recognized as a task.

Expectation

A list item whose task marker is followed by a tab renders as a task item, the same as when a space follows it:

marked.parse('- [x]\tfoo')
// <ul>\n<li><input checked="" disabled="" type="checkbox"> foo</li>\n</ul>\n

GitHub renders these as checkboxes. We checked - [ ]\tfoo, - [x]\tfoo and - [X] \titem (space, then tab) through GitHub's Markdown API (POST /markdown, mode: gfm) on 2026-10-24; each one returned an <input type="checkbox">.

The GFM spec (0.29, section 5.3) defines the marker as [, "either a whitespace character or the letter x", ], and section 2.1 defines a whitespace character as a space, tab, newline, line tabulation, form feed or carriage return. The spec does not say what has to follow the marker, and none of its examples has a tab after it. So we are relying on GitHub's own rendering here, not on a failing spec example.

Result

With gfm: true, the item stays literal text:

marked.parse('- [x]\tfoo')
// <ul>\n<li>[x]\tfoo</li>\n</ul>\n

The list item token has task: false and checked: undefined. The same happens after [ ] and [X], for space then tab, for tab then space, in ordered lists and in loose lists.

What was attempted

other.listIsTask (/^\[[ xX]\] +\S/) and other.listReplaceTask (/^\[[ xX]\] +/) in src/rules.ts accept only spaces after ]. This PR changes + to [ \t]+ in both. Nothing else changes.

These keep their current output: - [x] foo (task), - [x]foo (not a task), - [ ] and - [ ] followed by a tab and nothing else (not a task), and gfm: false (never a task). With the fix, the tab case renders <input ...> foo, the same as the space case does now; GitHub keeps the tab in the item text.

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.

New spec fixture test/specs/new/tasklist_tab.md / .html: a tab after [ ] and [x], and a space then a tab after [X]. The .md contains literal tab characters.

node --test test/run-spec-tests.js test/unit/*.test.js (the test:specs and test:unit suites) after npm run build:esbuild: without the src/rules.ts change only tasklist_tab fails; with it everything passes. eslint src/rules.ts is clean. We did not run test:umd, test:cjs, test:types or test:redos.

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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