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Fonts: a Thai fallback, so Thai text no longer draws as boxes (#1913)

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Thanks @DuBearte.
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DuBearte authored and GitHub committed 2026-10-17 04:10:18 -04:00
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commit 9c07bdd823
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+76 -13
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crates/ui-egui/src/fonts.rs
+76 -13
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@@ -1,9 +1,10 @@
//! System font loading and fallback setup for multi-language (CJK: Japanese, Chinese, Korean) support.
//! System font loading and fallback setup for multi-language (CJK: Japanese, Chinese, Korean;
//! Thai) support.
//!
//! The UI's Latin font always comes first; a CJK font (then a Hangul font) is only ever appended as
//! a fallback, so English text renders exactly as before and only glyphs the Latin font lacks use
//! the CJK face. The Hangul fallback covers Korean where the CJK font found (Meiryo, Hiragino)
//! lacks Hangul. The order of the CJK and Chinese faces follows the UI language
//! The UI's Latin font always comes first; a CJK font (then a Hangul font, then a Thai font) is only
//! ever appended as a fallback, so English text renders exactly as before and only glyphs the Latin
//! font lacks use the CJK face. The Hangul fallback covers Korean where the CJK font found (Meiryo,
//! Hiragino) lacks Hangul; the Thai fallback covers Thai typed in a name or the agent chat (#1902). The order of the CJK and Chinese faces follows the UI language
//! (see [`fallback_order`]): one face drawing a whole word keeps its glyphs on one baseline.
use std::sync::Arc;
@@ -44,9 +45,12 @@ pub fn with_ui_font(mut fonts: egui::FontDefinitions, font: egui::FontData) -> e
fonts
}
/// Name of the Hangul fallback entry in [`egui::FontDefinitions`], last.
/// Name of the Hangul fallback entry in [`egui::FontDefinitions`], after the CJK ones.
pub const HANGUL_FALLBACK: &str = "hangul_fallback";
/// Name of the Thai fallback entry in [`egui::FontDefinitions`], last (#1902).
pub const THAI_FALLBACK: &str = "thai_fallback";
/// UI languages whose text needs the CJK font. The web app's bundled font covers Japanese only;
/// add Chinese or Korean here together with a font that covers them.
pub const CJK_LANGUAGES: &[&str] = &["ja", "zh", "ko"];
@@ -318,6 +322,32 @@ const HANGUL_FONT_CANDIDATES: &[&str] = &[
"/usr/share/fonts/nanum/NanumGothic.ttf",
];
/// Thai fonts, appended last: none of the Latin or CJK faces draw Thai, so without one Thai text
/// shows as empty boxes (#1902).
#[cfg(not(target_arch = "wasm32"))]
const THAI_FONT_CANDIDATES: &[&str] = &[
// Windows: Leelawadee UI (Windows 8 and later), Leelawadee, Tahoma
"C:\\Windows\\Fonts\\LeelawUI.ttf",
"C:\\Windows\\Fonts\\leelawad.ttf",
"C:\\Windows\\Fonts\\tahoma.ttf",
// macOS: Thonburi, Ayuthaya
"/System/Library/Fonts/Thonburi.ttc",
"/System/Library/Fonts/Supplemental/Thonburi.ttc",
"/System/Library/Fonts/Supplemental/Ayuthaya.ttf",
"/Library/Fonts/Thonburi.ttf",
// Linux: Noto Sans Thai, TLWG Loma and Garuda
"/usr/share/fonts/noto/NotoSansThai-Regular.ttf",
"/usr/share/fonts/truetype/noto/NotoSansThai-Regular.ttf",
"/usr/share/fonts/google-noto/NotoSansThai-Regular.ttf",
"/usr/share/fonts/google-noto-vf/NotoSansThai[wght].ttf",
"/usr/share/fonts/opentype/noto/NotoSansThai-Regular.ttf",
"/usr/share/fonts/truetype/tlwg/Loma.ttf",
"/usr/share/fonts/opentype/tlwg/Loma.otf",
"/usr/share/fonts/truetype/tlwg/Garuda.ttf",
"/usr/share/fonts/opentype/tlwg/Garuda.otf",
"/usr/share/fonts/tlwg/Loma.otf",
];
/// Fonts covering simplified Chinese, appended after the CJK fallback so Chinese renders even
/// when the CJK font found is a Japanese one without simplified characters (Hiragino Gothic,
/// Meiryo, Yu Gothic).
@@ -464,11 +494,16 @@ fn find_system_cjk_font() -> Option<Vec<u8>> {
/// epaint centres each face's own line height in the row, so glyphs from two faces of very
/// different height (Meiryo ≈ 2.0 em, Microsoft YaHei ≈ 1.4 em) sit at different heights
/// inside one word. One face for the whole word keeps them on one baseline.
pub fn fallback_order(language: &str) -> [&'static str; 3] {
if language == "zh" { [CHINESE_FALLBACK, CJK_FALLBACK, HANGUL_FALLBACK] } else { [CJK_FALLBACK, CHINESE_FALLBACK, HANGUL_FALLBACK] }
/// Thai comes last whatever the language: no other face draws it.
pub fn fallback_order(language: &str) -> [&'static str; 4] {
if language == "zh" {
[CHINESE_FALLBACK, CJK_FALLBACK, HANGUL_FALLBACK, THAI_FALLBACK]
} else {
[CJK_FALLBACK, CHINESE_FALLBACK, HANGUL_FALLBACK, THAI_FALLBACK]
}
}
/// Configure font definitions with system CJK, Chinese and Hangul fallbacks where available,
/// Configure font definitions with system CJK, Chinese, Hangul and Thai fallbacks where available,
/// ordered for the UI language (see [`fallback_order`]), after the interface font `ui_font`
/// when one is chosen (see [`ui_font`]).
#[cfg(not(target_arch = "wasm32"))]
@@ -476,6 +511,7 @@ pub fn setup_fonts_for(ctx: &egui::Context, language: &str, ui_font: &str) {
let cjk = find_system_cjk_font();
let chinese = find_system_chinese_font().filter(|bytes| cjk.as_ref() != Some(bytes));
let hangul = HANGUL_FONT_CANDIDATES.iter().find_map(|path| try_read_font(std::path::Path::new(path)));
let thai = THAI_FONT_CANDIDATES.iter().find_map(|path| try_read_font(std::path::Path::new(path)));
let mut fonts = egui::FontDefinitions::default();
// Set the fonts also when only the interface font changes, so going back to the default
// takes the chosen one away.
@@ -487,7 +523,8 @@ pub fn setup_fonts_for(ctx: &egui::Context, language: &str, ui_font: &str) {
let bytes = match name {
CJK_FALLBACK => cjk.clone(),
CHINESE_FALLBACK => chinese.clone(),
_ => hangul.clone(),
HANGUL_FALLBACK => hangul.clone(),
_ => thai.clone(),
};
if let Some(bytes) = bytes
&& let Some(with_fallback) = with_fallback(fonts.clone(), name, bytes)
@@ -586,9 +623,13 @@ mod tests {
#[test]
fn a_chinese_ui_puts_the_chinese_face_before_the_japanese_one() {
assert_eq!(fallback_order("zh"), [CHINESE_FALLBACK, CJK_FALLBACK, HANGUL_FALLBACK]);
assert_eq!(fallback_order("zh"), [CHINESE_FALLBACK, CJK_FALLBACK, HANGUL_FALLBACK, THAI_FALLBACK]);
for lang in ["en", "ja", "ko", "de"] {
assert_eq!(fallback_order(lang), [CJK_FALLBACK, CHINESE_FALLBACK, HANGUL_FALLBACK], "{lang} keeps the Japanese font first");
assert_eq!(
fallback_order(lang),
[CJK_FALLBACK, CHINESE_FALLBACK, HANGUL_FALLBACK, THAI_FALLBACK],
"{lang} keeps the Japanese font first"
);
}
let defaults = egui::FontDefinitions::default();
let mut fonts = defaults.clone();
@@ -598,10 +639,32 @@ mod tests {
for family in [egui::FontFamily::Proportional, egui::FontFamily::Monospace] {
let list = &fonts.families[&family];
assert_eq!(list.first(), defaults.families[&family].first(), "the Latin UI font stays first");
assert_eq!(&list[list.len() - 3..], [CHINESE_FALLBACK, CJK_FALLBACK, HANGUL_FALLBACK]);
assert_eq!(&list[list.len() - 4..], [CHINESE_FALLBACK, CJK_FALLBACK, HANGUL_FALLBACK, THAI_FALLBACK]);
}
}
/// Thai typed into the agent chat drew as empty boxes (#1902): every Thai candidate this
/// machine has must parse and cover Thai, and the fonts set up must then draw it.
#[test]
fn a_thai_fallback_draws_thai() {
let found: Vec<Vec<u8>> = THAI_FONT_CANDIDATES.iter().filter_map(|path| try_read_font(std::path::Path::new(path))).collect();
for bytes in &found {
let face = ttf_parser::Face::parse(bytes, 0).unwrap();
for c in "สวัสดี".chars() {
assert!(face.glyph_index(c).is_some(), "{c:?} is in the Thai font");
}
}
let thai = |fonts: egui::FontDefinitions| {
let ctx = egui::Context::default();
ctx.set_fonts(fonts);
ctx.run_ui(egui::RawInput::default(), |_| {}).textures_delta.clear();
ctx.fonts_mut(|f| f.has_glyphs(&egui::FontId::proportional(14.0), "สวัสดี"))
};
assert!(!thai(egui::FontDefinitions::default()), "the built-in faces have no Thai");
let Some(bytes) = found.into_iter().next() else { return };
assert!(thai(with_fallback(egui::FontDefinitions::default(), THAI_FALLBACK, bytes).unwrap()), "Thai draws with the fallback");
}
#[test]
fn hangul_font_falls_back_after_the_cjk_font() {
let defaults = egui::FontDefinitions::default();