Detect system dark mode preference changes without restart on Linux

The new org.freedesktop.appearance.color-scheme key allows us to do this
in a reliable way.

Recently freedesktop standardized the system dark mode preference in a
desktop environment independent way in the xdg-desktop-portal
specification.

The specification can be seen here: https://github.com/flatpak/xdg-desktop-portal/blob/d7a304a00697d7d608821253cd013f3b97ac0fb6/data/org.freedesktop.impl.portal.Settings.xml#L33-L45

KDE supports this since KDE Plasma 5.24 and Gnome supports this since
Gnome 42.

Relevant blog post: https://blogs.gnome.org/alexm/2021/10/04/dark-style-preference/

Fixes #7146

src/gui/osutils/nixutils/NixUtils.cpp

@@ -18,6 +18,7 @@
 #include "NixUtils.h"
 
 #include <QApplication>
+#include <QDBusInterface>
 #include <QDir>
 #include <QPointer>
 #include <QStandardPaths>
@@ -60,6 +61,15 @@ NixUtils::NixUtils(QObject* parent)
 {
     dpy = QX11Info::display();
     rootWindow = QX11Info::appRootWindow();
+
+    // notify about system color scheme changes
+    QDBusConnection sessionBus = QDBusConnection::sessionBus();
+    sessionBus.connect("org.freedesktop.portal.Desktop",
+                       "/org/freedesktop/portal/desktop",
+                       "org.freedesktop.portal.Settings",
+                       "SettingChanged",
+                       this,
+                       SLOT(handleColorSchemeChanged(QString,QString,QDBusVariant)));
 }
 
 NixUtils::~NixUtils()
@@ -68,6 +78,11 @@ NixUtils::~NixUtils()
 
 bool NixUtils::isDarkMode() const
 {
+    // prefer freedesktop "org.freedesktop.appearance color-scheme" setting
+    if (m_systemColorschemePref != ColorschemePref::PreferNone) {
+        return m_systemColorschemePref == ColorschemePref::PreferDark;
+    }
+
     if (!qApp || !qApp->style()) {
         return false;
     }
@@ -257,3 +272,11 @@ bool NixUtils::unregisterGlobalShortcut(const QString& name)
     m_globalShortcuts.remove(name);
     return true;
 }
+
+void NixUtils::handleColorSchemeChanged(QString ns, QString key, QDBusVariant value)
+{
+    if (ns == "org.freedesktop.appearance" && key == "color-scheme") {
+        m_systemColorschemePref = static_cast<ColorschemePref>(value.variant().toInt());
+        emit interfaceThemeChanged();
+    }
+}

src/gui/osutils/nixutils/NixUtils.h

@@ -20,6 +20,7 @@
 
 #include "gui/osutils/OSUtilsBase.h"
 #include <QAbstractNativeEventFilter>
+#include <QtDBus/QDBusVariant>
 #include <QSharedPointer>
 
 class NixUtils : public OSUtilsBase, QAbstractNativeEventFilter
@@ -48,6 +49,9 @@ class NixUtils : public OSUtilsBase, QAbstractNativeEventFilter
         return false;
     }
 
+private slots:
+    void handleColorSchemeChanged(QString ns, QString key, QDBusVariant value);
+
 private:
     explicit NixUtils(QObject* parent = nullptr);
     ~NixUtils() override;
@@ -66,6 +70,15 @@ class NixUtils : public OSUtilsBase, QAbstractNativeEventFilter
     };
     QHash<QString, QSharedPointer<globalShortcut>> m_globalShortcuts;
 
+    // defined as per "org.freedesktop.appearance color-scheme" spec in
+    // https://github.com/flatpak/xdg-desktop-portal/blob/d7a304a00697d7d608821253cd013f3b97ac0fb6/data/org.freedesktop.impl.portal.Settings.xml#L33-L45
+    enum ColorschemePref {
+        PreferNone,
+        PreferDark,
+        PreferLight
+    };
+    ColorschemePref m_systemColorschemePref = ColorschemePref::PreferNone;
+
     Q_DISABLE_COPY(NixUtils)
 };