[cscheid]

Closes #892.

Problem

Task-list items in q2 render and q2 preview showed both the list bullet and the checkbox. Quarto 1 hides the bullet with a ul.task-list { list-style: none } rule from its patched Pandoc styles.html template. Q2 never ported that rule. It only ported the two _quarto-rules.scss rules.

Approach

Before choosing a fix, the triage surveyed about 20 markdown renderers and SSGs (claude-notes/issue-reports/892/survey.md, summary in triage.md). Nearly every renderer that ships CSS hides the marker per item. Pandoc, and therefore Quarto 1, hides it per list, so a mixed list still shows a bullet next to the checkbox. We follow the ecosystem. The design decisions are recorded in triage.md § Decisions:

1. Per-item marker hiding.
   • The HTML writer and q2-preview add class="task-list-item" to every task <li>. The class is additive, and ul.task-list keeps Pandoc's rule that every item must be a task.
   • The theme hides the marker on li.task-list-item and hangs the checkbox into the marker's gutter, so task text lines up with plain items.
2. Ordered task items keep their numbers by default. The $task-list-ordered-markers theme variable is an escape hatch: set it to false in a theme's scss:defaults layer. It works in both the HTML and revealjs themes and is documented in the themes guide.
3. An empty - [ ] is a task item. This is what a user has just typed in the collaborative editor. It used to be a parse error. The grammar now accepts it, the reader produces Pandoc's Plain [Str "☐"], and the qmd writer round-trips it. - [ ] with no trailing space is still an ordinary item, as in GFM.
4. Static checkboxes are disabled. Edit surfaces keep enabled, toggleable checkboxes.

Other fixes found along the way

• A hoisted item attribute (- [ ] todo {.c}) was dropped on task items. The writer now splits the attribute off before rewriting the task item.
• The incremental (revealjs) <ol> path in q2-preview didn't render task checkboxes. It now matches the <ul> path.

Review notes

• Grammar change. parser.c and grammar.json are regenerated. crates/pampa/resources/error-corpus/_autogen-table.json is regenerated with build_error_table.ts; parser state numbers shifted, and every table entry maps one-to-one onto a new state.
• Snapshots. No insta .snap files changed. The styles.css hash in the phase5 single-doc baseline was re-captured: this is a CSS-only change, doc.html is unchanged, and the reason is noted in the file.
• Follow-up, not in this PR. bd-nnvvomg6: the HTML writer drops <p> in loose list items that hold a single paragraph. This is general Pandoc parity work and also affects loose task lists.

Tests

• tree-sitter corpus: 2 new tests (empty task items).
• pampa test_task_list: 5 new, 3 updated (markup, empty item, attribute composition, qmd round-trip).
• quarto-sass: 3 new (marker rules, the escape hatch through a user custom.scss, revealjs), 1 updated.
• preview-renderer integration: 2 new, 3 extended. With the original renderer sources, 5 of these fail.
• hub-client e2e q2-preview-task-list.spec.ts (Chromium, real q2-preview iframe):
  • every task <li> computes list-style-type: none;
  • the checkbox sits left of the <li> box, and the text starts within 2px of the <li> content edge;
  • plain siblings in the mixed list keep their bullets.
• cargo xtask verify (full) passes locally.

End-to-end check (inspected): q2 render on claude-notes/issue-reports/892/repro.qmd emits <li class="task-list-item"><label><input type="checkbox" disabled="" />task</label></li> next to <li>plain item</li>. The compiled CSS contains ul>li.task-list-item{list-style-type:none} and the hang rule margin:0 .4em .2em -1.2em.

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