fix(qt): stabilize Japanese font fallback

cx

crates/qbz-qt/build.rs

@@ -317,8 +317,8 @@ fn build_rhi_items() {
         let moc = qtbuild.moc(&header, qt_build_utils::MocArguments::default());
         cc.file(source).file(&moc.cpp);
     }
-    // Plain helper, no QObject/moc: registers one script-specific bundled
-    // font through QFontDatabase before Main.qml constructs any text.
+    // Plain helper, no QObject/moc: registers script-specific bundled font
+    // fallbacks through QFontDatabase before Main.qml constructs any text.
     println!("cargo:rerun-if-changed=cxx/font_fallback.cpp");
     cc.file("cxx/font_fallback.cpp");
     // Plain helper, no QObject/moc: enumerates the exact QVulkanInstance that

crates/qbz-qt/cxx/font_fallback.cpp

@@ -73,3 +73,27 @@ extern "C" bool qbz_register_devanagari_fallback()
     installFontMessageFilter();
     return true;
 }
+
+extern "C" bool qbz_register_japanese_fallback()
+{
+    // LINE Seed JP is already bundled as a selectable UI/lyrics face.  Make
+    // it Qt's deterministic fallback for the three scripts used by Japanese
+    // text as well.  Without this, a Latin-only choice such as Source Sans 3
+    // delegates to the host's first fontconfig match; a user-installed
+    // MS Gothic can therefore receive synthetic Medium/Bold and render much
+    // heavier than the surrounding UI.
+    const auto id = QFontDatabase::addApplicationFont(QStringLiteral(
+        ":/qt/qml/com/blitzfc/qbz/qml/assets/fonts/"
+        "LINESeedJP-Regular.ttf"));
+    if (id < 0)
+        return false;
+
+    const auto families = QFontDatabase::applicationFontFamilies(id);
+    if (families.isEmpty())
+        return false;
+    const auto &family = families.constFirst();
+    QFontDatabase::addApplicationFallbackFontFamily(QChar::Script_Han, family);
+    QFontDatabase::addApplicationFallbackFontFamily(QChar::Script_Hiragana, family);
+    QFontDatabase::addApplicationFallbackFontFamily(QChar::Script_Katakana, family);
+    return true;
+}

crates/qbz-qt/qml/FontPreload.qml

@@ -20,8 +20,8 @@
 // Hence this document: `QQmlApplicationEngine::load` returns with its
 // FontLoader already in the font database (verified: the family is present
 // the instant load() returns), so main.rs can set the application font in the
-// gap between this document and Main.qml. "System" maps to an empty source:
-// no UI family is loaded and Qt keeps the operating-system default.
+// gap between this document and Main.qml. "System" leaves the Loader inactive:
+// no empty URL is assigned and Qt keeps the operating-system default.
 //
 // It is deliberately NON-VISUAL — a QtObject, not a Window — so loading it
 // adds a root object and nothing else.
@@ -46,7 +46,11 @@ QtObject {
             return "assets/fonts/SourceSans3-VariableFont_wght.ttf"
         return ""
     }
-    readonly property FontLoader selected: FontLoader {
-        source: root.selectedSource
+    readonly property Component selectedComponent: Component {
+        FontLoader { source: root.selectedSource }
+    }
+    readonly property Loader selected: Loader {
+        active: root.selectedSource !== ""
+        sourceComponent: root.selectedComponent
     }
 }

crates/qbz-qt/qml/immersive/LyricsFocusPanel.qml

@@ -58,17 +58,26 @@ Item {
         Math.min(root.width * 0.026, 48)))
 
     readonly property string fontDir: "qrc:/qt/qml/com/blitzfc/qbz/qml/assets/fonts/"
-    FontLoader { id: fLineSeed; source: QbzLyrics.fontIndex === 1 ? root.fontDir + "LINESeedJP-Regular.ttf" : "" }
-    FontLoader { id: fMontserrat; source: QbzLyrics.fontIndex === 2 ? root.fontDir + "Montserrat-VariableFont_wght.ttf" : "" }
-    FontLoader { id: fNoto; source: QbzLyrics.fontIndex === 3 ? root.fontDir + "NotoSans-VariableFont_wdth,wght.ttf" : "" }
-    FontLoader { id: fSource; source: QbzLyrics.fontIndex === 4 ? root.fontDir + "SourceSans3-VariableFont_wght.ttf" : "" }
-    readonly property string fontFamily: {
-        if (QbzLyrics.fontIndex === 1) return fLineSeed.status === FontLoader.Ready ? fLineSeed.name : ""
-        if (QbzLyrics.fontIndex === 2) return fMontserrat.status === FontLoader.Ready ? fMontserrat.name : ""
-        if (QbzLyrics.fontIndex === 3) return fNoto.status === FontLoader.Ready ? fNoto.name : ""
-        if (QbzLyrics.fontIndex === 4) return fSource.status === FontLoader.Ready ? fSource.name : ""
+    readonly property string fontSource: {
+        if (QbzLyrics.fontIndex === 1) return root.fontDir + "LINESeedJP-Regular.ttf"
+        if (QbzLyrics.fontIndex === 2) return root.fontDir + "Montserrat-VariableFont_wght.ttf"
+        if (QbzLyrics.fontIndex === 3) return root.fontDir + "NotoSans-VariableFont_wdth,wght.ttf"
+        if (QbzLyrics.fontIndex === 4) return root.fontDir + "SourceSans3-VariableFont_wght.ttf"
         return ""
     }
+    Component {
+        id: fontLoaderComponent
+        FontLoader { source: root.fontSource }
+    }
+    Loader {
+        id: selectedFontLoader
+        active: root.fontSource !== ""
+        sourceComponent: fontLoaderComponent
+    }
+    readonly property string fontFamily:
+        selectedFontLoader.item
+        && selectedFontLoader.item.status === FontLoader.Ready
+            ? selectedFontLoader.item.name : ""
 
     // --- Entry-animation retrigger (:57-74) --------------------------------
     readonly property int watchedIndex: root.activeIndex

crates/qbz-qt/qml/immersive/ReactiveSplitPanel.qml

@@ -86,17 +86,26 @@ Item {
         1.0 - root.ambientPrimary.b, 1.0), 1.18)
 
     readonly property string fontDir: "qrc:/qt/qml/com/blitzfc/qbz/qml/assets/fonts/"
-    FontLoader { id: fLineSeed; source: QbzLyrics.fontIndex === 1 ? root.fontDir + "LINESeedJP-Regular.ttf" : "" }
-    FontLoader { id: fMontserrat; source: QbzLyrics.fontIndex === 2 ? root.fontDir + "Montserrat-VariableFont_wght.ttf" : "" }
-    FontLoader { id: fNoto; source: QbzLyrics.fontIndex === 3 ? root.fontDir + "NotoSans-VariableFont_wdth,wght.ttf" : "" }
-    FontLoader { id: fSource; source: QbzLyrics.fontIndex === 4 ? root.fontDir + "SourceSans3-VariableFont_wght.ttf" : "" }
-    readonly property string lyricFontFamily: {
-        if (QbzLyrics.fontIndex === 1) return fLineSeed.status === FontLoader.Ready ? fLineSeed.name : ""
-        if (QbzLyrics.fontIndex === 2) return fMontserrat.status === FontLoader.Ready ? fMontserrat.name : ""
-        if (QbzLyrics.fontIndex === 3) return fNoto.status === FontLoader.Ready ? fNoto.name : ""
-        if (QbzLyrics.fontIndex === 4) return fSource.status === FontLoader.Ready ? fSource.name : ""
+    readonly property string fontSource: {
+        if (QbzLyrics.fontIndex === 1) return root.fontDir + "LINESeedJP-Regular.ttf"
+        if (QbzLyrics.fontIndex === 2) return root.fontDir + "Montserrat-VariableFont_wght.ttf"
+        if (QbzLyrics.fontIndex === 3) return root.fontDir + "NotoSans-VariableFont_wdth,wght.ttf"
+        if (QbzLyrics.fontIndex === 4) return root.fontDir + "SourceSans3-VariableFont_wght.ttf"
         return ""
     }
+    Component {
+        id: fontLoaderComponent
+        FontLoader { source: root.fontSource }
+    }
+    Loader {
+        id: selectedFontLoader
+        active: root.fontSource !== ""
+        sourceComponent: fontLoaderComponent
+    }
+    readonly property string lyricFontFamily:
+        selectedFontLoader.item
+        && selectedFontLoader.item.status === FontLoader.Ready
+            ? selectedFontLoader.item.name : ""
 
     function fmt(sec) {
         sec = Math.max(0, Math.floor(sec))

crates/qbz-qt/qml/shell/LyricsLinesView.qml

@@ -16,8 +16,8 @@
 //    moves of <= 2 lines, INSTANT for bigger jumps and the initial sync,
 //    1:1 with Slint. The animation drives `contentY` only on the auto-follow
 //    path, so user wheel input is never animated.
-//  - the four embedded lyric font families (lazy: a FontLoader only gets a
-//    source once its option is selected).
+//  - the embedded lyric font families (lazy: one FontLoader exists only while
+//    a non-System option is selected).
 //
 // Unsynced/plain documents render as a static top-aligned list: no spacers,
 // no active line, no scroll-follow.
@@ -60,23 +60,32 @@ Item {
 
     readonly property real contentWidth: Math.max(0, width - 2 * sidePadding)
 
-    // ---- embedded lyric fonts (lazy) -------------------------------------
+    // ---- embedded lyric font (lazy) --------------------------------------
     readonly property string fontDir: "qrc:/qt/qml/com/blitzfc/qbz/qml/assets/fonts/"
-    FontLoader { id: fLineSeed; source: view.fontIndex === 1 ? view.fontDir + "LINESeedJP-Regular.ttf" : "" }
-    FontLoader { id: fMontserrat; source: view.fontIndex === 2 ? view.fontDir + "Montserrat-VariableFont_wght.ttf" : "" }
-    FontLoader { id: fNoto; source: view.fontIndex === 3 ? view.fontDir + "NotoSans-VariableFont_wdth,wght.ttf" : "" }
-    FontLoader { id: fSource; source: view.fontIndex === 4 ? view.fontDir + "SourceSans3-VariableFont_wght.ttf" : "" }
+    readonly property string fontSource: {
+        if (view.fontIndex === 1) return view.fontDir + "LINESeedJP-Regular.ttf"
+        if (view.fontIndex === 2) return view.fontDir + "Montserrat-VariableFont_wght.ttf"
+        if (view.fontIndex === 3) return view.fontDir + "NotoSans-VariableFont_wdth,wght.ttf"
+        if (view.fontIndex === 4) return view.fontDir + "SourceSans3-VariableFont_wght.ttf"
+        return ""
+    }
+    Component {
+        id: fontLoaderComponent
+        FontLoader { source: view.fontSource }
+    }
+    Loader {
+        id: selectedFontLoader
+        active: view.fontSource !== ""
+        sourceComponent: fontLoaderComponent
+    }
 
     // "" = the real Qt/system default = Slint's "System" option. A family
     // that has not finished loading falls back to the default rather than
     // rendering in a wrong face.
-    readonly property string fontFamily: {
-        if (view.fontIndex === 1) return fLineSeed.status === FontLoader.Ready ? fLineSeed.name : ""
-        if (view.fontIndex === 2) return fMontserrat.status === FontLoader.Ready ? fMontserrat.name : ""
-        if (view.fontIndex === 3) return fNoto.status === FontLoader.Ready ? fNoto.name : ""
-        if (view.fontIndex === 4) return fSource.status === FontLoader.Ready ? fSource.name : ""
-        return ""
-    }
+    readonly property string fontFamily:
+        selectedFontLoader.item
+        && selectedFontLoader.item.status === FontLoader.Ready
+            ? selectedFontLoader.item.name : ""
 
     // ---- auto-center scroll ----------------------------------------------
     // Last index centered on; -1 = nothing yet -> the next center is INSTANT.

crates/qbz-qt/src/font_qt.rs

@@ -1,17 +1,24 @@
 //! Script-specific application font fallbacks.
 //!
-//! The UI's "System" choice remains Qt's real system font.  This module only
-//! gives Qt a deterministic face when a selected UI font lacks Devanagari,
-//! avoiding its noisy scan through unrelated installed families.
+//! The UI's "System" choice remains Qt's real system font for covered glyphs.
+//! This module only gives Qt deterministic bundled faces when that font (or a
+//! selected UI/lyrics font) lacks Japanese or Devanagari, avoiding arbitrary
+//! and sometimes visually incompatible host fallbacks.
 
 unsafe extern "C" {
     fn qbz_register_devanagari_fallback() -> bool;
+    fn qbz_register_japanese_fallback() -> bool;
 }
 
-pub(crate) fn register_devanagari_fallback() {
-    // SAFETY: the C++ function has no arguments, is called once on the GUI
-    // thread after QGuiApplication exists, and only mutates QFontDatabase's
+pub(crate) fn register_script_fallbacks() {
+    // SAFETY: these C++ functions have no arguments, are called once on the
+    // GUI thread after QGuiApplication exists, and only mutate QFontDatabase's
     // documented process-global application-font registry.
+    if unsafe { qbz_register_japanese_fallback() } {
+        log::info!("[qbz-qt] font fallback -> LINE Seed JP (Han/Hiragana/Katakana)");
+    } else {
+        log::warn!("[qbz-qt] bundled LINE Seed JP fallback failed to load");
+    }
     if unsafe { qbz_register_devanagari_fallback() } {
         log::info!("[qbz-qt] font fallback -> Noto Sans Devanagari");
     } else {

crates/qbz-qt/src/main.rs

@@ -3422,7 +3422,7 @@ fn main() {
             "qrc:/qt/qml/com/blitzfc/qbz/qml/FontPreload.qml",
         ));
     }
-    font_qt::register_devanagari_fallback();
+    font_qt::register_script_fallbacks();
     let app_font_family = settings_qt::app_font_family();
     if !app_font_family.is_empty() {
         if let Some(app) = app.as_mut() {