Text: a missing font's substitute takes fallback fonts across a colour change

Text is shaped in pieces that differ only in paint, so each piece group,
and each piece shaped apart, takes its style's environment: a missing
font's substitute draws the characters it lacks from fallback fonts
whatever Draw Missing Glyphs from Fallback Fonts says.

crates/compose/src/shape.rs

@@ -375,14 +375,17 @@ pub(crate) fn shape_para(
         let range = group[0].range.start..group[j - i - 1].range.end;
         let runs: Vec<(usize, u32)> = group.iter().map(|p| (p.range.start, p.style)).collect();
         let start = glyphs.len();
-        shape_run(db, &story.text, range, &first.props, auto_leading, &StyleRuns(&runs), sub, &mut glyphs);
+        // A missing font's substitute stands in for the whole font: fallback fonts help it.
+        // Pieces shaped together share their font, so the first one's says for all.
+        shape_run(db, &story.text, range, &first.props, table.env_for(auto_leading, first.style), &StyleRuns(&runs), sub, &mut glyphs);
         // A cluster (a ligature, a letter with its marks) that spans two styles would paint one
         // of them in the other's colour: shape those pieces apart instead.
         let spans_cut = |g: &Glyph| runs[1..].iter().any(|&(cut, _)| g.byte < cut && cut < g.byte + g.len);
         if group.len() > 1 && glyphs[start..].iter().any(spans_cut) {
             glyphs.truncate(start);
             for p in group {
-                shape_run(db, &story.text, p.range.clone(), &p.props, auto_leading, &StyleRuns(&[(p.range.start, p.style)]), sub, &mut glyphs);
+                let env = table.env_for(auto_leading, p.style);
+                shape_run(db, &story.text, p.range.clone(), &p.props, env, &StyleRuns(&[(p.range.start, p.style)]), sub, &mut glyphs);
             }
         }
         i = j;

crates/compose/src/tests.rs

@@ -1570,6 +1570,32 @@ fn missing_glyphs_are_the_fonts_box_unless_fallback_is_on() {
     assert_ne!(gid, 0, "a fallback font draws it");
 }
 
+/// A missing font's substitute draws what it lacks from fallback fonts whatever the setting, also
+/// in text shaped as one piece across a colour change.
+#[test]
+fn a_missing_fonts_substitute_takes_fallback_fonts() {
+    use designcraft_fonts::testing::font_with;
+    designcraft_fonts::FontDb::global().add_font(font_with("DC Test Missing Glyph Helper", &['語']).unwrap());
+    let text = "a語b語";
+    let (mut d, sid, _) = doc_with(text, Rect::new(36.0, 36.0, 300.0, 100.0), ParaAttrs::default());
+    {
+        let s = d.story_mut(sid).unwrap();
+        s.format_chars(0..text.len(), |f| {
+            f.over.language = Some("Japanese".into());
+            f.over.font_family = Some("DC Test No Such Family".into());
+        });
+        s.format_chars("a語".len()..text.len(), |f| f.over.fill = Some("Red".into()));
+    }
+    assert!(!d.settings.glyph_fallback);
+    let cs = compose_story(&d, sid, &ComposeOptions::default());
+    let glyphs: Vec<&PlacedGlyph> = all_lines(&cs).into_iter().flat_map(|l| l.glyphs.iter()).filter(|g| text[g.byte..].starts_with('語')).collect();
+    assert_eq!(glyphs.len(), 2);
+    for g in glyphs {
+        assert_ne!(g.face.family, designcraft_fonts::DEFAULT_FAMILY, "byte {}", g.byte);
+        assert_ne!(g.gid, 0, "byte {}: a fallback font draws it", g.byte);
+    }
+}
+
 #[test]
 fn kenten_missing_from_the_font_are_its_box_unless_fallback_is_on() {
     use designcraft_fonts::testing::font_with;