Fonts: order CJK fallbacks by UI language so a Chinese word keeps one baseline (#1070)

With a Chinese UI on Windows and macOS, the first CJK fallback was a Japanese font without the simplified characters, so one word mixed glyphs from two faces with different line heights and they sat on different baselines. For zh the Chinese face now joins the fallback chain before the Japanese one, and the chain is rebuilt on the frame the UI language changes; other languages keep the previous order.

crates/ui-egui/src/lib.rs

@@ -300,6 +300,9 @@ pub struct SolveApp {
     pub file_load: file_load::FileLoad,
     /// The other open designs (tabs).
     pub docs: documents::Documents,
+    /// The language the installed font fallback chain is ordered for (`fonts::fallback_order`);
+    /// changing `preferences.language` re-installs the fonts on the next frame.
+    applied_fonts_language: String,
 }
 
 impl SolveApp {
@@ -358,6 +361,7 @@ impl SolveApp {
             home: home::HomeState::default(),
             file_load: file_load::FileLoad::default(),
             docs: documents::Documents::default(),
+            applied_fonts_language: "en".into(),
         }
     }
 
@@ -1066,6 +1070,10 @@ impl SolveApp {
     pub fn ui(&mut self, ui: &mut egui::Ui) {
         expression_complete::begin_frame(ui.ctx());
         let t0 = now_ms();
+        if self.preferences.language != self.applied_fonts_language {
+            self.applied_fonts_language.clone_from(&self.preferences.language);
+            crate::fonts::setup_fonts_for(ui.ctx(), &self.preferences.language);
+        }
         let _language = crate::i18n::scope(&self.preferences.language);
         let mut report_input = None;
         issue_report::CadInput::block(self, ui, &mut report_input);