R18: a list label adds ascent, not descent, to its line (#928)

Word sizes a list item's first line with the label's ascent when it is
taller, but keeps the text's descent below the baseline: a Symbol bullet's
deeper descent (and a Courier New label's) doesn't count. Measured in Word
for Windows: 6 synthetic cases. NIH's bulleted items were a fifth of a point
taller each, enough to push lines over a page bottom.

Co-authored-by: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

ROADMAP.md

@@ -146,6 +146,7 @@ Ranked, to beta (detail and remaining milestones in [docs/roadmap.md](docs/roadm
 | 2026-10-12 | Line heights like Word's: spaces and tabs never make a line taller (13 cases measured in Word for Windows); tables of contents with big tab runs were a third taller |
 | 2026-10-12 | `.doc` import: tab stops (centred titles, right-aligned dates, leaders) and sections ending at their section marks (continuous breaks, footers); EPA .doc templates now paginate like Word |
 | 2026-10-12 | DOCX headings numbered like Word when a heading style gives only a list level (6.4.5, not 6.15) |
+| 2026-10-12 | A list label makes its line taller above the baseline only, as in Word (6 cases measured in Word for Windows): Symbol bullets no longer add their deeper descent, so bulleted lists keep Word's height and page breaks |
 | 2026-10-11 | Hide White Space (#645): `view.hideWhiteSpace` (View › Show checkbox, or double-click the gap between pages) collapses margins, headers and footers between pages to a thin line in Print Layout; screen only, page count unchanged |
 | 2026-10-11 | Fonts: Palatino Linotype and Book Antiqua fall back to Palatino (macOS) or TeX Gyre Pagella / P052 / URW Palladio (Linux) before generic serif (#670) |
 |---|---|

crates/layout/src/para.rs

@@ -1813,10 +1813,12 @@ fn break_lines(pl: &mut ParaLayout, env: &ParaEnv, mark_style: u16, level: Optio
             asc = st.ascent;
             desc = st.descent;
         }
+        // A list label can make the line taller above the baseline, never below it: Word keeps
+        // the text's descent (measured in Word for Windows: a Symbol bullet's taller ascent
+        // counts, a Courier New label's deeper descent doesn't).
         if first && let Some(lab) = &pl.label {
             for st in lab.runs.iter().filter_map(|r| pl.styles.get(r.style as usize)) {
                 asc = asc.max(st.ascent);
-                desc = desc.max(st.descent);
             }
         }
         // Line spacing scales the text's height, never a picture's: a line is at least as tall

crates/layout/src/tests.rs

@@ -4177,3 +4177,34 @@ fn spaces_and_tabs_do_not_make_a_line_taller() {
     assert!(pitch("x", &big) > plain + 5.0, "a 20 pt letter does make it taller");
     assert!(pitch("\u{A0}", &big) > plain + 5.0, "so does a no-break space");
 }
+
+/// A list label taller than its text makes the first line taller above the baseline only: Word
+/// keeps the text's descent below it (measured in Word for Windows, `plan/word` R18: a Symbol
+/// bullet's taller ascent counts, a Courier New label's deeper descent doesn't). NIH's bulleted
+/// items each came out a fifth of a point taller, enough to push a paragraph's last lines over
+/// the page bottom.
+#[test]
+fn a_list_label_adds_ascent_but_not_descent() {
+    let first_line = |label_size: Option<f32>| {
+        let mut d = Document::new();
+        let num = d.numbering.add_list(wordcraft_doc::ListKind::Numbered);
+        for a in &mut d.numbering.abstracts {
+            for l in &mut a.levels {
+                l.chr.size = label_size;
+            }
+        }
+        let ten = wordcraft_doc::props::CharProps { size: Some(10.0), ..Default::default() };
+        let mut p = wordcraft_doc::Paragraph::with_text("Item", ten.clone());
+        p.props.numbering = Some(wordcraft_doc::props::NumRef { num, level: 0 });
+        p.mark = ten;
+        p.props.line_spacing = Some(wordcraft_doc::props::LineSpacing::Multiple(1.0));
+        d.body = vec![wordcraft_doc::para_block(p)];
+        let l = lay(&d);
+        l.pages[0].items.iter().find_map(|it| if let Placed::Lines { para, .. } = it { para.lines.first().cloned() } else { None }).unwrap()
+    };
+    let plain = first_line(None);
+    let big = first_line(Some(30.0));
+    let below = |l: &crate::para::Line| l.top + l.height - l.baseline;
+    assert!(big.baseline - big.top > plain.baseline - plain.top + 5.0, "the big label raises the baseline");
+    assert!((below(&big) - below(&plain)).abs() < 0.01, "the text's descent: {} vs {}", below(&big), below(&plain));
+}

docs/layout-parity.md

@@ -76,6 +76,7 @@ font can't match Word's metrics.
 | Date | Change | Summary |
 |---|---|---|
 | 2026-10-12 | trivial | Spaces, tabs and ideographic spaces don't count toward a line's height, as in Word (a line of only them takes the paragraph mark's) |
+| 2026-10-12 | trivial | A list label's font raises the first line's ascent but never its descent (Word keeps the text's descent below the baseline) |
 | 2026-10-11 | trivial | Chinese documents (#534): indents in characters, a page-anchored floating table after a full page moves to the next page, no blank page from a page break at the bottom of a full page |
 | 2026-10-11 | trivial | East Asian list number formats (一、二、三, 第一章, ①); list labels whose characters come from two fonts draw each part with its own font (the digits of "1、" no longer show as other letters); a list level without indents leaves the label at the paragraph edge (#534) |
 | 2026-10-11 | trivial | Vertical sections: `w:textDirection` in `w:sectPr`, upright East Asian glyphs (#491) |