Text: superscript and subscript advance at their drawn size

Superscript and subscript glyphs were drawn at two thirds of the run's size but
advanced by the full size, so the letters spread apart ("s u p e r") and the
line ran wider than in PowerPoint. The cluster advance now uses the same scale
as the glyphs.

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

crates/text/src/lib.rs

@@ -566,6 +566,9 @@ fn shape_cells(ctx: &Ctx, text: &str, char_style: &[usize], styles: &[Style], fa
     cells
 }
 
+/// Size of superscript and subscript text relative to the run's size.
+const SUB_SUPER: f64 = 0.66;
+
 /// Shape cells `range` (one item) and hang each cluster's glyphs on its first cell, sharing the
 /// cluster's advance among its graphemes (by the ligature's GDEF carets when it has them).
 #[allow(clippy::too_many_arguments)]
@@ -593,7 +596,11 @@ fn shape_item(
         let st = styles.get(cell.style);
         let size = st.map(|s| s.size).unwrap_or(18.0);
         let small = st.is_some_and(|s| s.caps == Caps::Small) && cell.ch.is_lowercase();
-        (if small { size * 0.8 } else { size }) / upem
+        let size = if small { size * 0.8 } else { size };
+        // Superscript and subscript glyphs are drawn at two thirds of the size, so they advance by
+        // two thirds too.
+        let size = if st.is_some_and(|s| s.baseline != 0.0) { size * SUB_SUPER } else { size };
+        size / upem
     };
     // Cluster heads in this item, ascending; each cluster spans to the next head.
     let mut heads: Vec<usize> = glyphs.iter().filter_map(|g| char_at_byte.get(&g.cluster).copied()).filter(|h| range.contains(h)).collect();
@@ -1155,7 +1162,7 @@ fn layout_scaled(ctx: &Ctx, shape: &Shape, body: &TextBody, bp: &BodyProps, opts
             let Some(st) = p.styles.get(cell.style) else { continue };
             let Some(&(cx, cx1)) = line.edges.get(k) else { continue };
             let base_y = line.baseline - st.baseline * st.size;
-            let size = if st.baseline != 0.0 { st.size * 0.66 } else { st.size };
+            let size = if st.baseline != 0.0 { st.size * SUB_SUPER } else { st.size };
             let size = if st.caps == Caps::Small && cell.ch.is_lowercase() { size * 0.8 } else { size };
             let key = (cell.style, cell.face);
             if key != cur_key {
@@ -1204,9 +1211,8 @@ fn layout_scaled(ctx: &Ctx, shape: &Shape, body: &TextBody, bp: &BodyProps, opts
                             }
                         }
                     }
-                    let k66 = if st.baseline != 0.0 { 0.66 } else { 1.0 };
                     for (gid, gx, gy) in &cell.glyphs {
-                        r.glyphs.push((*gid, cl + kash + gx * k66, base_y + gy));
+                        r.glyphs.push((*gid, cl + kash + gx, base_y + gy));
                         r.cells.push((ci, cell.ch));
                     }
                 }

crates/text/src/tests.rs

@@ -395,3 +395,41 @@ fn hostile_bidi_input_never_panics() {
         }
     }
 }
+
+/// Width of `word` laid out in a run with `baseline` (0 = normal, 0.3 = superscript …).
+fn run_width(word: &str, baseline: Option<f64>) -> f64 {
+    let mut run = Run::new(word);
+    run.props.size = Some(24.0);
+    run.props.baseline = baseline;
+    let (p, sh) = bidi_setup(vec![run], 2000.0, false, Align::Left);
+    let l = lay(&p, &sh, 2000.0, 300.0);
+    let line = &l.lines[0];
+    line.caret_x.last().unwrap() - line.caret_x.first().unwrap()
+}
+
+#[test]
+fn superscript_and_subscript_advance_at_their_drawn_size() {
+    // The glyphs are drawn at two thirds of the size; advancing by the full size spread the
+    // letters apart ("s u p e r").
+    let normal = run_width("superscript", None);
+    for b in [Some(0.3), Some(-0.25)] {
+        let w = run_width("superscript", b);
+        assert!((w / normal - SUB_SUPER).abs() < 0.02, "{b:?}: {w} vs {normal}");
+    }
+}
+
+#[test]
+fn superscript_glyphs_sit_inside_their_cells() {
+    let mut run = Run::new("super");
+    run.props.size = Some(24.0);
+    run.props.baseline = Some(0.3);
+    let (p, sh) = bidi_setup(vec![run], 2000.0, false, Align::Left);
+    let l = lay(&p, &sh, 2000.0, 300.0);
+    let line = &l.lines[0];
+    let r = l.runs.iter().find(|r| !r.cells.is_empty()).unwrap();
+    assert!((r.size - 24.0 * SUB_SUPER).abs() < 1e-6);
+    for (g, (ci, _)) in r.glyphs.iter().zip(&r.cells) {
+        let (x0, x1) = line.edges[*ci - line.start];
+        assert!(g.1 >= x0 - 0.01 && g.1 < x1, "glyph at {} outside its cell {x0}..{x1}", g.1);
+    }
+}