Merge feat/layout-parity update: Layout: a baseline shift doesn't move the Em Box Height first baseline

crates/compose/src/lib.rs

@@ -2601,14 +2601,16 @@ fn line_metrics(
     let reference = src.iter().max_by(|a, b| a.size.total_cmp(&b.size));
     let mut own_text = false;
     for g in src {
-        let shift = g.shift + reference.map_or(0.0, |r| cjk_alignment_shift(g, r));
+        let align = reference.map_or(0.0, |r| cjk_alignment_shift(g, r));
+        let shift = g.shift + align;
         let up = shift.max(0.0);
         asc = asc.max(g.ascent + up);
         // A list's bullet or number doesn't raise the Ascent offset: only the line's own text does.
         if !g.list_label {
             own_text = true;
             tops.typo_ascent = tops.typo_ascent.max(g.typo_ascent + up);
-            tops.em = tops.em.max(em_box_top(&g.face, g.size) + up);
+            // The em box sits at the unshifted baseline: a baseline shift moves only the drawn glyphs.
+            tops.em = tops.em.max(em_box_top(&g.face, g.size) + align.max(0.0));
         }
         tops.cap = tops.cap.max(g.cap + up);
         tops.xh = tops.xh.max(g.xh + up);

crates/compose/src/tests.rs

@@ -2451,6 +2451,32 @@ fn em_box_first_baseline_in_a_latin_font_with_a_one_em_typo_box_follows_the_cap_
     assert!((first - expected).abs() < 0.01, "{first}, expected {expected}");
 }
 
+#[test]
+fn em_box_first_baseline_ignores_baseline_shift() {
+    use designcraft_doc::FirstBaseline;
+    let db = designcraft_fonts::FontDb::global();
+    // Source Sans 3's `BASE` em box tops 0.83 em above the baseline.
+    db.add_font(test_font("EmBoxShft 830", Some(&[])));
+    let (mut d, sid, fid) = doc_with("Hxg Hxg", Rect::new(36.0, 36.0, 300.0, 300.0), ParaAttrs::default());
+    let st = d.story_mut(sid).unwrap();
+    st.format_chars(0..7, |f| {
+        f.over.font_family = Some("EmBoxShft 830".into());
+        f.over.font_style = Some("Regular".into());
+        f.over.size = Some(10.0);
+    });
+    st.format_chars(4..7, |f| f.over.baseline_shift = Some(1.0));
+    d.item_mut(fid).unwrap().text_frame_mut().unwrap().options.first_baseline = FirstBaseline::EmboxHeight;
+    let frames = compose_story(&d, sid, &ComposeOptions::default()).frames;
+    let line = &frames[0].lines[0];
+    // The unshifted run's baseline stays 8.3 pt below the frame top; the shifted run is drawn 1 pt
+    // higher.
+    assert!((line.baseline - 36.0 - 8.3).abs() < 0.01, "{}", line.baseline);
+    let ys: Vec<f64> = line.glyphs.iter().map(|g| g.y).collect();
+    assert_eq!(ys.len(), 7, "{ys:?}");
+    assert!(ys[..3].iter().all(|y| y.abs() < 0.01), "{ys:?}");
+    assert!(ys[4..].iter().all(|y| (y + 1.0).abs() < 0.01), "{ys:?}");
+}
+
 #[test]
 fn ascent_first_baseline_falls_back_to_the_ascent_without_a_usable_typo_ascender() {
     let db = designcraft_fonts::FontDb::global();