Type: Top-to-Top Leading — leading measured from em box top to em box top

Japanese layout measures leading from the top of one line's ideographic
em box to the next (the right side, in vertical type), not from baseline
to baseline. The Paragraph panel menu now offers both:

- `ParaStyle.leading_model`: RomanBaseline (as before, the default) or
  EmBoxTop. With EmBoxTop a line's leading is the space below it — the
  next line's em box top is this line's plus its leading, and the next
  baseline hangs from there by that line's tallest em box — and area
  type's first line touches the top of the frame (its em box top on the
  frame's top, at least Area Type Options' minimum). With baseline
  leading a line's leading stays the space above it.
- Lines of one size are exactly as far apart either way; the models
  differ for mixed sizes and for the first line of area type (fonts
  whose ascent isn't the em box top: Hiragino's nearly is).
- The em box top comes from the font's vertical metrics (#343), else
  0.88 em. Written only when EmBoxTop.
- `text.setFormat {leadingModel: "romanBaseline"|"emBoxTop"}`; menu items
  Top-to-Top Leading / Bottom-to-Bottom Leading (checked as set); catalog
  rows in ja and zh-hant.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/engine/src/cmd/panelcmds.rs

@@ -24,7 +24,7 @@ pub fn specs() -> Vec<CommandSpec> {
             "Character / Paragraph",
             [],
             None,
-            "{ids?|id?, kerning?: 1/1000 em|\"auto\", baselineShift?: pt, hScale?: %, vScale?: %, rotation?: deg, underline?, strikethrough?, allCaps?, smallCaps?: bool, position?: \"normal\"|\"superscript\"|\"subscript\" (sizes from Document Setup), leftIndent?, rightIndent?, firstLineIndent?, spaceBefore?, spaceAfter?: pt, hyphenate?: bool, mojikumi?: \"none\"|\"lineEndHalf\" (Japanese punctuation spacing), direction?: \"auto\"|\"leftToRight\"|\"rightToLeft\" (paragraph direction; auto: from each paragraph's first strong character)}",
+            "{ids?|id?, kerning?: 1/1000 em|\"auto\", baselineShift?: pt, hScale?: %, vScale?: %, rotation?: deg, underline?, strikethrough?, allCaps?, smallCaps?: bool, position?: \"normal\"|\"superscript\"|\"subscript\" (sizes from Document Setup), leftIndent?, rightIndent?, firstLineIndent?, spaceBefore?, spaceAfter?: pt, hyphenate?: bool, mojikumi?: \"none\"|\"lineEndHalf\" (Japanese punctuation spacing), direction?: \"auto\"|\"leftToRight\"|\"rightToLeft\" (paragraph direction; auto: from each paragraph's first strong character), leadingModel?: \"romanBaseline\"|\"emBoxTop\" (leading measured baseline to baseline, or em box top to top)}",
             has_doc,
             set_format
         ),
@@ -113,6 +113,7 @@ fn set_format(s: &mut Session, p: &Value) -> Result<Value> {
         "hyphenate",
         "mojikumi",
         "direction",
+        "leadingModel",
     ];
     if !keys.iter().any(|k| p.get(*k).is_some()) {
         return Err(bad(C, "nothing to change"));
@@ -135,6 +136,14 @@ fn set_format(s: &mut Session, p: &Value) -> Result<Value> {
             _ => return Err(bad(C, "`direction` must be \"auto\", \"leftToRight\" or \"rightToLeft\"")),
         }),
     };
+    let leading_model = match p.get("leadingModel") {
+        None => None,
+        Some(v) => Some(match v.as_str() {
+            Some("romanBaseline") => vectorcraft_doc::LeadingModel::RomanBaseline,
+            Some("emBoxTop") => vectorcraft_doc::LeadingModel::EmBoxTop,
+            _ => return Err(bad(C, "`leadingModel` must be \"romanBaseline\" or \"emBoxTop\"")),
+        }),
+    };
     s.edit("Character", |d, _| {
         for id in &ids {
             let Some(NodeKind::Text(t)) = d.node_mut(*id).map(|n| &mut n.kind) else { continue };
@@ -196,6 +205,9 @@ fn set_format(s: &mut Session, p: &Value) -> Result<Value> {
             if let Some(v) = direction {
                 para.direction = v;
             }
+            if let Some(v) = leading_model {
+                para.leading_model = v;
+            }
             super::typecmd::refresh_bounds(t);
         }
         Ok(())

crates/engine/src/cmd/typecmd.rs

@@ -408,6 +408,31 @@ fn reshape_area(s: &mut Session, p: &Value) -> Result<Value> {
 mod area_tests {
     use super::*;
 
+    #[test]
+    fn leading_model_is_set_per_paragraph_and_saved_only_when_top_to_top() {
+        use vectorcraft_doc::LeadingModel;
+        let mut s = Session::new();
+        s.execute("file.new", &json!({"width": 400, "height": 400})).unwrap();
+        let id = s.execute("text.create", &json!({"x": 10, "y": 10, "text": "一\n二", "area": {"width": 200, "height": 100}})).unwrap()["id"]
+            .as_u64()
+            .unwrap();
+        let text = |s: &Session| match &s.doc().unwrap().doc.node(NodeId(id)).unwrap().kind {
+            NodeKind::Text(t) => (**t).clone(),
+            _ => panic!("text"),
+        };
+        assert_eq!(text(&s).para.leading_model, LeadingModel::RomanBaseline);
+        s.execute("select.set", &json!({"ids": [id]})).unwrap();
+        assert!(s.execute("text.setFormat", &json!({"leadingModel": "middle"})).is_err());
+        let before = text(&s).cached_bounds;
+        s.execute("text.setFormat", &json!({"leadingModel": "emBoxTop"})).unwrap();
+        let t = text(&s);
+        assert_eq!(t.para.leading_model, LeadingModel::EmBoxTop);
+        assert_ne!(t.cached_bounds, before, "the first line moves up to the frame's top");
+        assert_eq!(serde_json::to_value(&t.para).unwrap()["leadingModel"], "emBoxTop");
+        s.execute("edit.undo", &json!({})).unwrap();
+        assert!(serde_json::to_value(&text(&s).para).unwrap().get("leadingModel").is_none());
+    }
+
     #[test]
     fn new_type_is_composed_with_line_end_half_width_punctuation() {
         use vectorcraft_doc::Mojikumi;