Type: Thai falls back to a font made for Thai

Thai typed in a font without Thai glyphs (Source Sans 3, a Latin font)
took the first loaded face that had them, or the first installed file
that did, in path order: on one Windows machine a display face (Kanit)
rather than Leelawadee UI. Characters of Thai, Lao, Khmer and Myanmar
now try fonts made for their script first, installed ones loaded on
demand (Leelawadee UI, Tahoma, Thonburi, Sukhumvit Set, Noto Sans Thai,
the TLWG fonts for Thai), then the fallback as before. Latin beside
them keeps the font chosen.

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

crates/text/src/fontdb.rs

@@ -391,6 +391,36 @@ struct SysFallback {
     misses: std::collections::HashSet<char>,
 }
 
+/// Fonts made for Thai, tried first (when installed) for Thai the font in use lacks: Windows
+/// (Leelawadee UI; Tahoma before Windows 8), macOS (Thonburi, Sukhumvit Set) and Linux (Noto,
+/// TLWG).
+pub const THAI_FAMILIES: &[&str] = &[
+    "Leelawadee UI",
+    "Leelawadee",
+    "Tahoma",
+    "Thonburi",
+    "Sukhumvit Set",
+    "Noto Sans Thai",
+    "Noto Sans Thai UI",
+    "Noto Sans Thai Looped",
+    "Waree",
+    "Loma",
+    "Garuda",
+];
+
+/// The families tried first for character `c`, in a script whose text only fonts made for it set
+/// well (Thai, Lao, Khmer, Myanmar); none for other characters.
+fn script_families(c: char) -> &'static [&'static str] {
+    use unicode_script::{Script, UnicodeScript};
+    match c.script() {
+        Script::Thai => THAI_FAMILIES,
+        Script::Lao => &["Lao UI", "Leelawadee UI", "Lao Sangam MN", "Noto Sans Lao", "Noto Sans Lao UI", "Phetsarath OT"],
+        Script::Khmer => &["Khmer UI", "Khmer Sangam MN", "Noto Sans Khmer", "Noto Sans Khmer UI", "Khmer OS"],
+        Script::Myanmar => &["Myanmar Text", "Myanmar Sangam MN", "Noto Sans Myanmar", "Noto Sans Myanmar UI", "Padauk"],
+        _ => &[],
+    }
+}
+
 /// Families tried (when installed) for characters the loaded fonts lack: CJK, symbols, emoji.
 #[cfg(not(target_arch = "wasm32"))]
 const SYSTEM_FALLBACKS: &[&str] = &[
@@ -1917,6 +1947,18 @@ impl FontDb {
     /// First face (fallback family first, then load order) that covers `c`; on native, system
     /// fonts are loaded lazily the first time no loaded face covers a character.
     pub(crate) fn fallback_for(&self, c: char, exclude: u32) -> Option<Arc<FontFace>> {
+        // Thai (Lao, Khmer, Myanmar) takes a font made for it, when one is installed, rather than
+        // whichever face covering it happened to load first (a display face, say).
+        let prefer = script_families(c);
+        if let Some(f) = self.family_fallback(c, exclude, prefer) {
+            return Some(f);
+        }
+        #[cfg(not(target_arch = "wasm32"))]
+        if self.load_families(c, prefer)
+            && let Some(f) = self.family_fallback(c, exclude, prefer)
+        {
+            return Some(f);
+        }
         if let Some(f) = self.loaded_fallback(c, exclude) {
             return Some(f);
         }
@@ -1934,6 +1976,28 @@ impl FontDb {
         self.fallback_for(c, 0)
     }
 
+    /// The loaded face of the first of `families` that covers `c` (upright, nearest Regular).
+    fn family_fallback(&self, c: char, exclude: u32, families: &[&str]) -> Option<Arc<FontFace>> {
+        let faces = self.read_faces();
+        families.iter().find_map(|fam| {
+            faces
+                .iter()
+                .filter(|f| f.id != exclude && f.family.eq_ignore_ascii_case(fam) && f.covers(c))
+                .min_by_key(|f| (f.italic, (f.weight - 400.0).abs() as i32))
+                .cloned()
+        })
+    }
+
+    /// Load the first of the installed `families` not loaded yet that covers `c` (with system
+    /// fallback on). Returns whether one was loaded.
+    #[cfg(not(target_arch = "wasm32"))]
+    fn load_families(&self, c: char, families: &[&str]) -> bool {
+        if families.is_empty() || !self.sys.lock().unwrap_or_else(|e| e.into_inner()).enabled {
+            return false;
+        }
+        families.iter().any(|fam| !self.is_loaded(fam) && self.load_cataloged(fam) && self.family_fallback(c, 0, &[fam]).is_some())
+    }
+
     fn loaded_fallback(&self, c: char, exclude: u32) -> Option<Arc<FontFace>> {
         let faces = self.read_faces();
         let mut order: Vec<&Arc<FontFace>> = faces.iter().filter(|f| f.id != exclude).collect();

crates/text/src/tests_sysfonts.rs

@@ -569,3 +569,23 @@ fn family_display_name_follows_show_font_names_in_english() {
     let face = db.face("ヒラギノ角ゴシック", "Regular").unwrap();
     assert_eq!(face.family, "Hiragino Sans");
 }
+
+/// Thai typed in a font without Thai takes an installed font made for Thai (Leelawadee UI,
+/// Thonburi, Noto Sans Thai…), whatever other face covering Thai loaded first; Latin beside it keeps
+/// the font chosen. Skipped where none of those families is installed.
+#[test]
+fn thai_falls_back_to_a_font_made_for_thai() {
+    let db = FontDb::global();
+    let Some(thai) = crate::fontdb::THAI_FAMILIES.iter().find(|f| db.has_family(f)) else {
+        eprintln!("skipped: no Thai font installed");
+        return;
+    };
+    let st = vectorcraft_doc::CharStyle { size: 20.0, ..Default::default() };
+    let l = layout(db, &TextObject::point(kurbo::Point::ZERO, "Thai ภาษาไทย", st));
+    let family = |g: &PositionedGlyph| db.face_by_id(g.font_id).map(|f| f.family.clone()).unwrap_or_default();
+    let (latin, thai_glyphs) = l.glyphs.split_at(5);
+    assert!(latin.iter().all(|g| family(g) == FALLBACK_FAMILY), "{:?}", latin.iter().map(family).collect::<Vec<_>>());
+    let first = crate::fontdb::THAI_FAMILIES.iter().find(|f| thai_glyphs.iter().all(|g| family(g).eq_ignore_ascii_case(f)));
+    assert!(first.is_some(), "Thai drawn in {:?}; {thai} is installed", thai_glyphs.iter().map(family).collect::<Vec<_>>());
+    assert!(thai_glyphs.iter().all(|g| g.gid != 0), "no .notdef");
+}

docs/type-parity.md

@@ -1,6 +1,6 @@
 # Type parity
 
-> **Last reviewed:** 2026-10-10 · **Last updated:** 2026-10-11 · **Change:** minor (Proportional Metrics in vertical type, `vpal`, #966; document-point character controls after object/group scaling, #1034) · **Target:** Adobe Illustrator 2026 (30.x)
+> **Last reviewed:** 2026-10-10 · **Last updated:** 2026-10-11 · **Change:** minor (Thai typed in a font without Thai falls back to a font made for Thai; earlier: Proportional Metrics in vertical type, `vpal`, #966) · **Target:** Adobe Illustrator 2026 (30.x)
 
 Type is Illustrator's largest single area by use (weight 13 of 105 across "Type core" and "Type advanced" in
 [target-app-parity.md](target-app-parity.md)). This checklist lists the features one by one. Status is from the
@@ -27,6 +27,7 @@ against Illustrator's public documentation; **estimated** unless noted.
 | Preserve font sizes while scaling a group | missing | Separate enhancement; scaling still scales text with the other artwork. |
 | OpenType panel, Glyphs panel, alternates | done | Highlight Alternate Glyphs preference unused (#394) |
 | Font menu: samples, live preview, filters, favourites; font matching by any name; Find Font | done | #339 |
+| Missing glyphs set in an installed font made for their script (Thai, Lao, Khmer, Myanmar first; then any font that has them) | done | Thai: Leelawadee UI, Tahoma, Thonburi, Sukhumvit Set, Noto Sans Thai, TLWG |
 | Variable fonts | partial | named instances as styles; axis sliders missing |
 | Colour fonts (OpenType SVG / COLR) | partial | |
 | Every-line and Single-line composers | done | tuned by eye, never compared side by side |
@@ -51,6 +52,7 @@ against Illustrator's public documentation; **estimated** unless noted.
 
 | Date | Change | Summary |
 |---|---|---|
+| 2026-10-11 | minor | Thai (Lao, Khmer, Myanmar) missing from the font in use falls back to an installed font made for the script instead of whichever face covering it loaded first; score unchanged (estimated) |
 | 2026-10-11 | minor | Proportional Metrics done: `palt` in horizontal type, `vpal` in vertical type (#966, #1082); ruby and manual tate-chu-yoko still missing |
 | 2026-10-10 | minor | #1034: transformed character controls and regression coverage; preserving group font sizes remains a separate enhancement. Scores/hours unchanged at this rounding. |
 | 2026-10-10 | major | First checklist, split out of the parity table's Type core and Type advanced rows |