fix(qbz-qt): honor system font with script fallback

cx

crates/qbz-qt/build.rs

@@ -317,6 +317,10 @@ 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.
+    println!("cargo:rerun-if-changed=cxx/font_fallback.cpp");
+    cc.file("cxx/font_fallback.cpp");
     for p in qtbuild.include_paths() {
         cc.include(p);
     }

crates/qbz-qt/cxx/font_fallback.cpp

@@ -0,0 +1,75 @@
+#include <QtCore/QChar>
+#include <QtCore/QByteArray>
+#include <QtCore/QMessageLogContext>
+#include <QtCore/QString>
+#include <QtCore/QStringList>
+#include <QtCore/qlogging.h>
+#include <QtGui/QFontDatabase>
+
+#include <cstdio>
+
+namespace {
+
+QtMessageHandler previousMessageHandler = nullptr;
+bool messageFilterInstalled = false;
+
+bool isExpectedDevanagariCapabilityWarning(
+    QtMsgType type, const QMessageLogContext &context, const QString &message)
+{
+    return type == QtWarningMsg
+        && context.category != nullptr
+        && qstrcmp(context.category, "qt.text.font.db") == 0
+        && message.startsWith(QStringLiteral("OpenType support missing for \""))
+        && message.endsWith(QStringLiteral(", script 11"));
+}
+
+void fontMessageHandler(
+    QtMsgType type, const QMessageLogContext &context, const QString &message)
+{
+    // Qt probes the selected UI face before consulting the application
+    // fallback. A face without Devanagari tables therefore emits this benign
+    // diagnostic even though the registered Noto face renders the text. Keep
+    // every other Qt diagnostic intact.
+    if (isExpectedDevanagariCapabilityWarning(type, context, message))
+        return;
+
+    if (previousMessageHandler != nullptr) {
+        previousMessageHandler(type, context, message);
+        return;
+    }
+
+    const auto rendered = qFormatLogMessage(type, context, message).toLocal8Bit();
+    std::fwrite(rendered.constData(), 1, rendered.size(), stderr);
+    std::fputc('\n', stderr);
+    std::fflush(stderr);
+}
+
+void installFontMessageFilter()
+{
+    if (messageFilterInstalled)
+        return;
+    previousMessageHandler = qInstallMessageHandler(fontMessageHandler);
+    messageFilterInstalled = true;
+}
+
+} // namespace
+
+extern "C" bool qbz_register_devanagari_fallback()
+{
+    // This is an application font, not the UI font.  Register it only for
+    // Devanagari so Qt never has to walk (and warn about) unrelated system
+    // families when catalog metadata contains that script.
+    const auto id = QFontDatabase::addApplicationFont(QStringLiteral(
+        ":/qt/qml/com/blitzfc/qbz/qml/assets/fonts/"
+        "NotoSansDevanagari-VariableFont_wght.ttf"));
+    if (id < 0)
+        return false;
+
+    const auto families = QFontDatabase::applicationFontFamilies(id);
+    if (families.isEmpty())
+        return false;
+    QFontDatabase::addApplicationFallbackFontFamily(
+        QChar::Script_Devanagari, families.constFirst());
+    installFontMessageFilter();
+    return true;
+}

crates/qbz-qt/qml/FontPreload.qml

@@ -1,4 +1,4 @@
-// FontPreload — registers the bundled typefaces BEFORE any Text exists.
+// FontPreload — registers the SELECTED bundled typeface before any Text exists.
 //
 // WHY THIS FILE IS A SEPARATE DOCUMENT, loaded before Main.qml.
 //
@@ -18,26 +18,35 @@
 // is only registered once something loads its file.
 //
 // Hence this document: `QQmlApplicationEngine::load` returns with its
-// FontLoaders already in the font database (verified: the family is present
+// 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.
+// 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.
 //
 // It is deliberately NON-VISUAL — a QtObject, not a Window — so loading it
 // adds a root object and nothing else.
 //
-// The five faces are settings_qt::APP_FONT_VALUES, in that order. Inter is
-// here too: it is the app's own face, named by Main.qml for the Controls, and
-// it needs registering for exactly the same reason.
+// The four non-system faces are settings_qt::APP_FONT_VALUES[1..], in order.
+// Noto Sans Devanagari is registered separately by font_qt as a script-only
+// fallback; it is never an application-font selection.
 
 import QtQuick
+import com.blitzfc.qbz
 
 QtObject {
-    readonly property FontLoader inter: FontLoader { source: "assets/fonts/Inter_18pt-Regular.ttf" }
-    readonly property FontLoader interMedium: FontLoader { source: "assets/fonts/Inter_18pt-Medium.ttf" }
-    readonly property FontLoader interSemiBold: FontLoader { source: "assets/fonts/Inter_18pt-SemiBold.ttf" }
-    readonly property FontLoader interBold: FontLoader { source: "assets/fonts/Inter_18pt-Bold.ttf" }
-    readonly property FontLoader lineSeed: FontLoader { source: "assets/fonts/LINESeedJP-Regular.ttf" }
-    readonly property FontLoader montserrat: FontLoader { source: "assets/fonts/Montserrat-VariableFont_wght.ttf" }
-    readonly property FontLoader notoSans: FontLoader { source: "assets/fonts/NotoSans-VariableFont_wdth,wght.ttf" }
-    readonly property FontLoader sourceSans3: FontLoader { source: "assets/fonts/SourceSans3-VariableFont_wght.ttf" }
+    id: root
+    readonly property string selectedSource: {
+        if (QbzShell.appFontFamily === "LINE Seed JP")
+            return "assets/fonts/LINESeedJP-Regular.ttf"
+        if (QbzShell.appFontFamily === "Montserrat")
+            return "assets/fonts/Montserrat-VariableFont_wght.ttf"
+        if (QbzShell.appFontFamily === "Noto Sans")
+            return "assets/fonts/NotoSans-VariableFont_wdth,wght.ttf"
+        if (QbzShell.appFontFamily === "Source Sans 3")
+            return "assets/fonts/SourceSans3-VariableFont_wght.ttf"
+        return ""
+    }
+    readonly property FontLoader selected: FontLoader {
+        source: root.selectedSource
+    }
 }

crates/qbz-qt/qml/Main.qml

@@ -3,9 +3,9 @@
 // sequence once the QML tree is complete (the bridge registers its Qt
 // thread handle in that first invokable, so every async UI hop lands).
 //
-// Font: the Slint app bundles the Inter 18pt faces (app.slint); the same
-// TTFs are embedded here via qrc and applied app-wide through the
-// ApplicationWindow font (children inherit).
+// Font: main.rs applies an explicitly selected bundled family before this
+// document is built. With "System" it applies nothing, so both plain Text and
+// Qt Quick Controls inherit the operating-system font chosen by Qt.
 
 import QtQuick
 import QtQuick.Controls
@@ -173,29 +173,6 @@ ApplicationWindow {
     rightPadding: 0
     color: "#1a1a1a"
 
-    FontLoader { id: interRegular; source: "assets/fonts/Inter_18pt-Regular.ttf" }
-    FontLoader { id: interMedium; source: "assets/fonts/Inter_18pt-Medium.ttf" }
-    FontLoader { id: interSemiBold; source: "assets/fonts/Inter_18pt-SemiBold.ttf" }
-    FontLoader { id: interBold; source: "assets/fonts/Inter_18pt-Bold.ttf" }
-    // The quality badge's face in the Tauri build — loading it here registers
-    // the family app-wide, so QualityBadge can name it without carrying its
-    // own loader into every row it renders in.
-    FontLoader { id: lineSeed; source: "assets/fonts/LINESeedJP-Regular.ttf" }
-
-    // Settings > Appearance > Typography & Language > Font — the CONTROLS half.
-    //
-    // The app's own text does not come from here: a plain `Text` takes the
-    // APPLICATION font at construction, which main() sets before this document
-    // is even loaded (see qml/FontPreload.qml for the measurements). What this
-    // line still owns is Qt Quick Controls, which follow ApplicationWindow.font
-    // and would otherwise sit on Inter while every label around them moved.
-    //
-    // "" = System, i.e. leave Qt's own choice alone, which is what this app
-    // rendered before the setting existed.
-    font.family: QbzShell.appFontFamily !== ""
-        ? QbzShell.appFontFamily
-        : (interRegular.status === FontLoader.Ready ? interRegular.name : "Sans Serif")
-
     // Phase 23: every domain singleton boots (registers its Qt-thread
     // hop; QbzSession.boot additionally fires the app boot sequence).
     // ── The renderer probe ────────────────────────────────────────────────

crates/qbz-qt/qml/settings/AppearanceSettings.qml

@@ -239,14 +239,12 @@ Column {
             onSelected: function (i) { QbzBridge.settingsSelect("ui-scale", i) }
         }
     }
-    // The app-wide typeface. The families are the ones already BUNDLED —
-    // the same set the lyrics panel offers (shell/LyricsControlsFlyout.qml)
-    // and the same mechanism the Tauri build had — so picking one costs no
-    // download and cannot fail on a machine that lacks the face. "System"
-    // means the window default, which is Inter (Main.qml).
+    // The app-wide typeface. The four named families are bundled — the same
+    // set the lyrics panel offers (shell/LyricsControlsFlyout.qml) — while
+    // "System" deliberately leaves Qt's operating-system default untouched.
     SettingRow {
         label: QbzSession.tr("Font", QbzSession.trRev)
-        description: QbzSession.tr("The typeface used across the app. Only fonts shipped with QBZ are offered, so the choice looks the same on every machine (requires restart)", QbzSession.trRev)
+        description: QbzSession.tr("The typeface used across the app. System follows your operating system; the other choices are bundled with QBZ (requires restart)", QbzSession.trRev)
         QbzSelect {
             menuWidth: 200
             options: root.doc.appFonts || []

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

@@ -68,7 +68,7 @@ Column {
 
     QbzTheme { id: theme }
 
-    // "" = the window default (Inter) — Qt resets an empty family to the
+    // "" = the Qt/system default — Qt resets an empty family to the
     // application font, exactly Slint's empty font-family = "System" option.
     readonly property string effFamily: row.fontFamily
 

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

@@ -68,7 +68,7 @@ Item {
     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" : "" }
 
-    // "" = the window default (Inter) = Slint's "System" option. A family
+    // "" = 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: {

crates/qbz-qt/src/font_qt.rs

@@ -0,0 +1,20 @@
+//! 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.
+
+unsafe extern "C" {
+    fn qbz_register_devanagari_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
+    // documented process-global application-font registry.
+    if unsafe { qbz_register_devanagari_fallback() } {
+        log::info!("[qbz-qt] font fallback -> Noto Sans Devanagari");
+    } else {
+        log::warn!("[qbz-qt] bundled Noto Sans Devanagari fallback failed to load");
+    }
+}

crates/qbz-qt/src/main.rs

@@ -71,6 +71,7 @@ mod playlist_picker_bridge;
 // QbzFolderEdit and the shared playlist editor on QbzPlaylistEdit — so a
 // folder save cannot perturb an open playlist editor.
 mod folder_edit_bridge;
+mod font_qt;
 mod playlist_edit_bridge;
 mod playlist_manager_bridge;
 // Purchases (2026-08-15 purchases contract §G.1): the QbzPurchases singleton —
@@ -3249,6 +3250,7 @@ fn main() {
             "qrc:/qt/qml/com/blitzfc/qbz/qml/FontPreload.qml",
         ));
     }
+    font_qt::register_devanagari_fallback();
     let app_font_family = settings_qt::app_font_family();
     if !app_font_family.is_empty() {
         if let Some(app) = app.as_mut() {

crates/qbz-qt/src/settings_qt.rs

@@ -1426,13 +1426,11 @@ const UI_SCALE_VALUES: &[&str] = &["xs", "small", "default", "large", "xl"];
 /// The app-wide typeface (Settings > Appearance > Typography & Language).
 ///
 /// The SAME five the lyrics panel offers (`lyrics_qt::FONTS`) and the same
-/// slugs, because they are the same files: everything here is BUNDLED in
-/// `qml/assets/fonts/`, so a choice cannot fail on a machine that does not
-/// have the face installed, and it looks identical everywhere. That is the
-/// Tauri build's mechanism, which is what the owner asked for.
+/// slugs. The four named families are bundled in `qml/assets/fonts/`, so an
+/// explicit choice cannot fail on a machine that lacks the face. "System" is
+/// the exception: it deliberately leaves Qt's operating-system choice alone.
 ///
 /// Only "System" is translated — a typeface name is a proper noun.
-/// "System" resolves to the window default, which is Inter (Main.qml).
 const APP_FONT_LABELS: &[&str] = &[
     "System",
     "LINE Seed JP",

crates/qbz-qt/third-party/LICENSE-Noto-Sans-Devanagari.txt

@@ -0,0 +1,93 @@
+Copyright 2022 The Noto Project Authors (https://github.com/notofonts/devanagari)
+
+This Font Software is licensed under the SIL Open Font License, Version 1.1.
+This license is copied below, and is also available with a FAQ at:
+https://scripts.sil.org/OFL
+
+
+-----------------------------------------------------------
+SIL OPEN FONT LICENSE Version 1.1 - 26 February 2007
+-----------------------------------------------------------
+
+PREAMBLE
+The goals of the Open Font License (OFL) are to stimulate worldwide
+development of collaborative font projects, to support the font creation
+efforts of academic and linguistic communities, and to provide a free and
+open framework in which fonts may be shared and improved in partnership
+with others.
+
+The OFL allows the licensed fonts to be used, studied, modified and
+redistributed freely as long as they are not sold by themselves. The
+fonts, including any derivative works, can be bundled, embedded,
+redistributed and/or sold with any software provided that any reserved
+names are not used by derivative works. The fonts and derivatives,
+however, cannot be released under any other type of license. The
+requirement for fonts to remain under this license does not apply
+to any document created using the fonts or their derivatives.
+
+DEFINITIONS
+"Font Software" refers to the set of files released by the Copyright
+Holder(s) under this license and clearly marked as such. This may
+include source files, build scripts and documentation.
+
+"Reserved Font Name" refers to any names specified as such after the
+copyright statement(s).
+
+"Original Version" refers to the collection of Font Software components as
+distributed by the Copyright Holder(s).
+
+"Modified Version" refers to any derivative made by adding to, deleting,
+or substituting -- in part or in whole -- any of the components of the
+Original Version, by changing formats or by porting the Font Software to a
+new environment.
+
+"Author" refers to any designer, engineer, programmer, technical
+writer or other person who contributed to the Font Software.
+
+PERMISSION & CONDITIONS
+Permission is hereby granted, free of charge, to any person obtaining
+a copy of the Font Software, to use, study, copy, merge, embed, modify,
+redistribute, and sell modified and unmodified copies of the Font
+Software, subject to the following conditions:
+
+1) Neither the Font Software nor any of its individual components,
+in Original or Modified Versions, may be sold by itself.
+
+2) Original or Modified Versions of the Font Software may be bundled,
+redistributed and/or sold with any software, provided that each copy
+contains the above copyright notice and this license. These can be
+included either as stand-alone text files, human-readable headers or
+in the appropriate machine-readable metadata fields within text or
+binary files as long as those fields can be easily viewed by the user.
+
+3) No Modified Version of the Font Software may use the Reserved Font
+Name(s) unless explicit written permission is granted by the corresponding
+Copyright Holder. This restriction only applies to the primary font name as
+presented to the users.
+
+4) The name(s) of the Copyright Holder(s) or the Author(s) of the Font
+Software shall not be used to promote, endorse or advertise any
+Modified Version, except to acknowledge the contribution(s) of the
+Copyright Holder(s) and the Author(s) or with their explicit written
+permission.
+
+5) The Font Software, modified or unmodified, in part or in whole,
+must be distributed entirely under this license, and must not be
+distributed under any other license. The requirement for fonts to
+remain under this license does not apply to any document created
+using the Font Software.
+
+TERMINATION
+This license becomes null and void if any of the above conditions are
+not met.
+
+DISCLAIMER
+THE FONT SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTIES OF
+MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT
+OF COPYRIGHT, PATENT, TRADEMARK, OR OTHER RIGHT. IN NO EVENT SHALL THE
+COPYRIGHT HOLDER BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
+INCLUDING ANY GENERAL, SPECIAL, INDIRECT, INCIDENTAL, OR CONSEQUENTIAL
+DAMAGES, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+FROM, OUT OF THE USE OR INABILITY TO USE THE FONT SOFTWARE OR FROM
+OTHER DEALINGS IN THE FONT SOFTWARE.

crates/qbz-qt/third-party/README.md

@@ -20,3 +20,13 @@ bundles them instead. The whole set is ~159 KB.
 registering — dropping a file in is enough. That is also why this licence
 lives HERE and not beside the SVGs: the sweep takes every file it finds, and
 a markdown file has no business being a QML resource.
+
+## Noto Sans Devanagari (SIL OFL 1.1)
+
+`qml/assets/fonts/NotoSansDevanagari-VariableFont_wght.ttf` — the hinted,
+weight-variable slim build from Noto Sans Devanagari v2.006. Source archive:
+https://github.com/notofonts/devanagari.
+
+QBZ registers this family only as Qt's application fallback for Devanagari;
+it is not an interface-font option and never replaces the operating-system
+font selected by “System”. Licence: `LICENSE-Noto-Sans-Devanagari.txt`.