[kcms/style] Consider GTK theme setting when computing default state

The GTK theme name is not managed by KConfigXT, we need to manually track that when determining whether the Defaults button is enabled

BUG: 458292

kcms/style/gtkpage.cpp

@@ -100,5 +100,11 @@ void GtkPage::defaults()
 void GtkPage::load()
 {
     m_gtkThemesModel->load();
+    m_gtkThemesModel->setSelectedTheme(m_gtkConfigInterface.gtkTheme());
     Q_EMIT selectGtkThemeInCombobox(gtkThemeFromConfig());
 }
+
+bool GtkPage::isDefaults() const
+{
+    return m_gtkThemesModel->selectedTheme() == QLatin1String("Breeze");
+}

kcms/style/gtkpage.h

@@ -26,6 +26,7 @@ public:
     Q_INVOKABLE void load();
     void save();
     void defaults();
+    bool isDefaults() const;
 
 public Q_SLOTS:
     QString gtkThemeFromConfig();

kcms/style/kcmstyle.cpp

@@ -86,19 +86,18 @@ KCMStyle::KCMStyle(QObject *parent, const KPluginMetaData &data, const QVariantL
     connect(styleSettings(), &StyleSettings::iconsInMenusChanged, this, [this] {
         m_effectsDirty = true;
     });
+
+    m_gtkPage = new GtkPage(this);
+    connect(m_gtkPage, &GtkPage::gtkThemeSettingsChanged, this, [this]() {
+        settingsChanged();
+        setNeedsSave(true);
+    });
 }
 
 KCMStyle::~KCMStyle() = default;
 
-GtkPage *KCMStyle::gtkPage()
+GtkPage *KCMStyle::gtkPage() const
 {
-    if (!m_gtkPage) {
-        m_gtkPage = new GtkPage(this);
-        connect(m_gtkPage, &GtkPage::gtkThemeSettingsChanged, this, [this]() {
-            setNeedsSave(true);
-        });
-    }
-
     return m_gtkPage;
 }
 
@@ -248,9 +247,7 @@ void KCMStyle::load()
 {
     checkGtkConfigKdedModuleLoaded();
 
-    if (m_gtkPage) {
-        m_gtkPage->load();
-    }
+    m_gtkPage->load();
 
     ManagedConfigModule::load();
     m_model->load();
@@ -263,9 +260,7 @@ void KCMStyle::load()
 
 void KCMStyle::save()
 {
-    if (m_gtkPage) {
-        m_gtkPage->save();
-    }
+    m_gtkPage->save();
 
     // Check whether the new style can actually be loaded before saving it.
     // Otherwise apps will use the default style despite something else having been written to the config
@@ -319,9 +314,7 @@ void KCMStyle::save()
 
 void KCMStyle::defaults()
 {
-    if (m_gtkPage) {
-        m_gtkPage->defaults();
-    }
+    m_gtkPage->defaults();
 
     // TODO the old code had a fallback chain but do we actually support not having Breeze for Plasma?
     // defaultStyle() -> oxygen -> plastique -> windows -> platinum -> motif
@@ -340,4 +333,9 @@ void KCMStyle::loadSettingsToModel()
     setOtherToolBarStyle(static_cast<ToolBarStyle>(toolBarStyleEnum.keyToValue(qUtf8Printable(styleSettings()->toolButtonStyleOtherToolbars()))));
 }
 
+bool KCMStyle::isDefaults() const
+{
+    return m_gtkPage->isDefaults();
+}
+
 #include "kcmstyle.moc"

kcms/style/kcmstyle.h

@@ -52,7 +52,7 @@ public:
     };
     Q_ENUM(ToolBarStyle)
 
-    GtkPage *gtkPage();
+    GtkPage *gtkPage() const;
 
     StylesModel *model() const;
 
@@ -75,6 +75,8 @@ public:
     void save() override;
     void defaults() override;
 
+    bool isDefaults() const override;
+
 Q_SIGNALS:
     void showErrorMessage(const QString &message);
     void styleReconfigured(const QString &styleName);