Apply new platform palette without overriding app palette

Currently when a palette is set by qt6ct, it can in some cases override
an app-specific palette. It would be nice if qt6ct would only set the
platform palette (i.e. QPlatformTheme::SystemPalette) and still allow
apps to set/adjust their own palette when desired.

Test case: start Audacious and select Dark theme in Audacious settings.
Then in qt6ct configuration, select "dusk" or "sand" color scheme. It
partially applies to Audacious (where it should not) and you end up with
an unusable mix of Audacious's color scheme and qt6ct's.

Port of the qt5ct patch from https://sourceforge.net/p/qt5ct/tickets/97/.
I have been using the patch in both qt5ct and qt6ct for quite some time
and have not seen any ill side effects.

trialuser02#48

src/qt6ct-qtplugin/qt6ctplatformtheme.cpp

@@ -92,8 +92,9 @@ QPlatformDialogHelper *Qt6CTPlatformTheme::createPlatformDialogHelper(DialogType
 
 const QPalette *Qt6CTPlatformTheme::palette(QPlatformTheme::Palette type) const
 {
-    qDebug() << Q_FUNC_INFO << type;
-    return (m_usePalette && m_palette) ? m_palette.get() : QGenericUnixTheme::palette(type);
+    if (type == QPlatformTheme::SystemPalette && !m_isIgnored)
+        return &m_palette;
+    return QPlatformTheme::palette(type);
 }
 
 const QFont *Qt6CTPlatformTheme::font(QPlatformTheme::Font type) const
@@ -152,21 +153,10 @@ void Qt6CTPlatformTheme::applySettings()
 {
     if(!QGuiApplication::desktopSettingsAware() || m_isIgnored)
     {
-        m_usePalette = false;
         m_update = true;
         return;
     }
 
-    if(!m_update)
-    {
-        //do not override application palette
-        if(QCoreApplication::testAttribute(Qt::AA_SetPalette))
-        {
-            m_usePalette = false;
-            qCDebug(lqt6ct) << "palette support is disabled";
-        }
-    }
-
     QGuiApplication::setFont(m_generalFont); //apply font
 
 #ifdef QT_WIDGETS_LIB
@@ -182,12 +172,6 @@ void Qt6CTPlatformTheme::applySettings()
             Qt6CT::reloadStyleInstanceSettings();
         }
 
-        if(!m_palette)
-            m_palette = std::make_unique<QPalette>(qApp->style()->standardPalette());
-
-        if(m_update && m_usePalette)
-            qApp->setPalette(*m_palette);
-
         if(m_userStyleSheet != m_prevStyleSheet)
         {
             // prepend our stylesheet to that of the application
@@ -209,7 +193,10 @@ void Qt6CTPlatformTheme::applySettings()
 #endif
 
     if(m_update)
+    {
         QIconLoader::instance()->updateSystemTheme(); //apply icons
+        QGuiApplication::setPalette(QGuiApplication::palette()); //apply palette
+    }
 
 #ifdef QT_WIDGETS_LIB
     if(hasWidgets() && m_update)
@@ -218,8 +205,6 @@ void Qt6CTPlatformTheme::applySettings()
         {
             QEvent e(QEvent::ThemeChange);
             QApplication::sendEvent(w, &e);
-            if(m_palette && m_usePalette)
-                w->setPalette(*m_palette);
         }
     }
 #endif
@@ -250,17 +235,16 @@ void Qt6CTPlatformTheme::updateSettings()
 
 void Qt6CTPlatformTheme::readSettings()
 {
-    m_palette.reset();
-
     QSettings settings(Qt6CT::configFile(), QSettings::IniFormat);
 
     settings.beginGroup("Appearance");
     m_style = settings.value("style", "Fusion").toString();
+    m_palette = *QPlatformTheme::palette(SystemPalette);
     QString schemePath = settings.value("color_scheme_path").toString();
     if(!schemePath.isEmpty() && settings.value("custom_palette", false).toBool())
     {
         schemePath = Qt6CT::resolvePath(schemePath); //replace environment variables
-        m_palette = std::make_unique<QPalette>(Qt6CT::loadColorScheme(schemePath, *QPlatformTheme::palette(SystemPalette)));
+        m_palette = Qt6CT::loadColorScheme(schemePath, m_palette);
     }
     m_iconTheme = settings.value("icon_theme").toString();
     //load dialogs
@@ -338,8 +322,6 @@ void Qt6CTPlatformTheme::readSettings()
             QCoreApplication::setAttribute(Qt::AA_ForceRasterWidgets, true);
         else if(!m_isIgnored && forceRasterWidgets == Qt::Unchecked)
             QCoreApplication::setAttribute(Qt::AA_ForceRasterWidgets, false);
-        if(m_isIgnored)
-            m_usePalette = false;
         settings.endGroup();
     }
 }

src/qt6ct-qtplugin/qt6ctplatformtheme.h

@@ -80,15 +80,14 @@ private slots:
 #endif
     QString loadStyleSheets(const QStringList &paths);
     QString m_style, m_iconTheme, m_userStyleSheet, m_prevStyleSheet;
-    std::unique_ptr<QPalette> m_palette;
+    QPalette m_palette;
     QFont m_generalFont, m_fixedFont;
     int m_doubleClickInterval;
     int m_cursorFlashTime;
     int m_uiEffects;
     int m_buttonBoxLayout;
     int m_keyboardScheme;
     bool m_update = false;
-    bool m_usePalette = true;
     int m_toolButtonStyle = Qt::ToolButtonFollowStyle;
     int m_wheelScrollLines = 3;
     bool m_showShortcutsInContextMenus = false;