M4: Thai faces in the text fallback list

Thai typed into a layer whose font has no Thai glyphs (Inter, which new
text starts in) was drawn as missing-glyph boxes, on the desktop and on
the web, even with a Thai face embedded or installed: FALLBACK_FAMILIES
listed Latin, CJK, Hangul, Arabic and symbol families but no Thai one.

Noto Sans Thai, Noto Sans Thai UI, Leelawadee UI, Thonburi and Tahoma go
after Noto Sans Arabic; they are only consulted for characters the
layer's own font lacks. Tests: the list has a Thai face for each
platform, and with one installed or embedded every Thai cluster in a
Latin layer, marks included, is drawn from it (skips without one).

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

crates/text/src/behaviour_tests.rs

@@ -265,6 +265,35 @@ fn hangul_comes_from_a_korean_font_when_one_is_installed() {
     }
 }
 
+#[test]
+fn thai_comes_from_a_thai_font_when_one_is_installed() {
+    // The default (Latin) face has no Thai. With a Thai face (Noto Sans Thai from craft-fonts or
+    // Linux, Leelawadee UI or Tahoma on Windows, Thonburi on macOS) the fallback draws every Thai
+    // cluster, marks included, instead of the missing-glyph box. Skips on a machine without one.
+    let Some(f) = face() else { return };
+    if f.has_char('\u{0E01}') {
+        return;
+    }
+    let thai = ["Noto Sans Thai", "Noto Sans Thai UI", "Leelawadee UI", "Thonburi", "Tahoma"];
+    if !thai.iter().any(|t| find_face(t, "Regular").is_some_and(|t| t.has_char('\u{0E01}'))) {
+        return;
+    }
+    // "ภาษาไทย" (the Thai language) and "น้ำ" (water: a tone mark over SARA AM).
+    let s = "Ab \u{0E20}\u{0E32}\u{0E29}\u{0E32}\u{0E44}\u{0E17}\u{0E22} \u{0E19}\u{0E49}\u{0E33}";
+    let chars: Vec<char> = s.chars().collect();
+    let l = layout(&text(s, TextAutoResize::WidthAndHeight), None, None);
+    let is_thai = |g: &&Glyph| chars.get(g.char_start).is_some_and(|c| ('\u{0E00}'..='\u{0E7F}').contains(c));
+    let glyphs: Vec<_> = l.glyphs.iter().filter(is_thai).collect();
+    assert!(!glyphs.is_empty());
+    for g in &glyphs {
+        assert!(g.gid != 0, "char {}: missing glyph in {} {}", g.char_start, g.face.family, g.face.style);
+        assert!(g.face.has_char('\u{0E01}'), "char {}: drawn by {}, which has no Thai", g.char_start, g.face.family);
+    }
+    let drawn: std::collections::BTreeSet<usize> = glyphs.iter().flat_map(|g| g.char_start..g.char_end).collect();
+    let want: std::collections::BTreeSet<usize> = (3..chars.len()).filter(|i| chars[*i] != ' ').collect();
+    assert_eq!(drawn, want, "every Thai char is in a drawn cluster");
+}
+
 #[test]
 fn break_opportunities() {
     // (before, after, may break between, derivation)

crates/text/src/fontdb.rs

@@ -406,7 +406,7 @@ fn names(f: &skrifa::FontRef) -> (Option<String>, Option<String>) {
 }
 
 /// Families tried, in order, for characters a text's own font lacks.
-const FALLBACK_FAMILIES: [&str; 20] = [
+const FALLBACK_FAMILIES: [&str; 25] = [
     "Inter",
     "Helvetica Neue",
     "Arial",
@@ -425,6 +425,11 @@ const FALLBACK_FAMILIES: [&str; 20] = [
     "Noto Sans CJK KR",
     "Noto Sans KR",
     "Noto Sans Arabic",
+    "Noto Sans Thai",
+    "Noto Sans Thai UI",
+    "Leelawadee UI",
+    "Thonburi",
+    "Tahoma",
     "Apple Symbols",
     "Segoe UI Symbol",
 ];

crates/text/src/fontdb_tests.rs

@@ -263,3 +263,15 @@ fn font_bytes_load_into_the_db() {
         assert_eq!(db().read().unwrap().face_count(), faces);
     }
 }
+
+#[test]
+fn every_platform_has_a_thai_font_in_the_fallbacks() {
+    // Thai typed into a Latin layer (new text starts in Inter) is drawn by the first fallback that
+    // has it: Noto Sans Thai (craft-fonts, Linux), Leelawadee UI or Tahoma (Windows), Thonburi
+    // (macOS). They come after the Latin faces, which go on drawing Latin text.
+    let at = |family: &str| FALLBACK_FAMILIES.iter().position(|f| *f == family);
+    let latin = at("Segoe UI").unwrap();
+    for thai in ["Noto Sans Thai", "Leelawadee UI", "Tahoma", "Thonburi"] {
+        assert!(at(thai).is_some_and(|i| i > latin), "{thai}");
+    }
+}