Compose: an empty paragraph's format comes from its own mark

Story::char_format_at returns the preceding run at a boundary (the format
typing takes), so a run ending at the empty paragraph's start gave it the
previous paragraph's mark format. Add Story::char_format_of (the run
containing the byte) and use it; regression test with differing marks.

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

crates/compose/src/lib.rs

@@ -650,7 +650,7 @@ pub fn compose(doc: &Document, story: &Story, frames: &[FrameSpec], opts: &Compo
         let spacing = spacing_for(&pp, base_size);
         // An empty paragraph takes its height from its paragraph mark's character format.
         let mark_chars = if glyphs.is_empty() {
-            std::borrow::Cow::Owned(doc.styles.resolve_char(&base_chars, story.char_format_at(prange.start)))
+            std::borrow::Cow::Owned(doc.styles.resolve_char(&base_chars, story.char_format_of(prange.start)))
         } else {
             std::borrow::Cow::Borrowed(&base_chars)
         };

crates/compose/src/tests.rs

@@ -1044,3 +1044,22 @@ fn empty_paragraph_takes_its_marks_format() {
     assert!((lines[1].baseline - lines[0].baseline - 8.0).abs() < 1e-6, "{} {}", lines[0].baseline, lines[1].baseline);
     assert!((lines[2].baseline - lines[1].baseline - 8.0).abs() < 1e-6, "{} {}", lines[1].baseline, lines[2].baseline);
 }
+
+#[test]
+fn empty_paragraph_uses_its_own_mark_not_the_previous_one() {
+    // "One\n" is 20 pt; the empty paragraph's mark (byte 4) and "Two" are 6 pt with 8 pt leading.
+    // The format run boundary sits exactly at the empty paragraph's start: its line must take the
+    // 8 pt leading of its own mark, not the previous paragraph's 20 pt (auto 24) mark.
+    let text = "One\n\nTwo";
+    let (mut d, sid, _) = doc_with(text, Rect::new(0.0, 0.0, 200.0, 200.0), ParaAttrs::default());
+    let st = d.story_mut(sid).unwrap();
+    st.format_chars(0..4, |f| f.over.size = Some(20.0));
+    st.format_chars(4..text.len(), |f| {
+        f.over.size = Some(6.0);
+        f.over.leading = Some(designcraft_doc::Leading::Points(8.0));
+    });
+    let cs = compose_story(&d, sid, &ComposeOptions::default());
+    let lines = all_lines(&cs);
+    assert_eq!(lines.len(), 3);
+    assert!((lines[1].baseline - lines[0].baseline - 8.0).abs() < 1e-6, "{} {}", lines[0].baseline, lines[1].baseline);
+}

crates/doc/src/story.rs

@@ -192,6 +192,21 @@ impl Story {
         self.chars.last().map_or(&NO_FORMAT, |r| &r.format)
     }
 
+    /// Format of the character starting at byte `pos` (the run containing `pos`), unlike
+    /// [`Self::char_format_at`], which at a run boundary returns the format of the character
+    /// before the caret. Past the end, the last run's format.
+    pub fn char_format_of(&self, pos: usize) -> &CharFormat {
+        let mut start = 0usize;
+        for r in &self.chars {
+            let end = start.saturating_add(r.len);
+            if pos < end {
+                return &r.format;
+            }
+            start = end;
+        }
+        self.chars.last().map_or(&NO_FORMAT, |r| &r.format)
+    }
+
     /// Iterate `(range, format)` of character runs.
     pub fn runs(&self) -> impl Iterator<Item = (std::ops::Range<usize>, &CharFormat)> {
         let mut start = 0;