It's enabled in qt6 by default

app/main.cpp

@@ -154,8 +154,10 @@ int main(int argc, char *argv[])
     /**
      * enable high dpi support
      */
+#if QT_VERSION < QT_VERSION_CHECK(6, 0, 0)
     QCoreApplication::setAttribute(Qt::AA_UseHighDpiPixmaps, true);
     QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling, true);
+#endif
     QCoreApplication::setAttribute(Qt::AA_CompressHighFrequencyEvents, true);
 
     QApplication app(argc, argv);

importer/main.cpp

@@ -38,8 +38,9 @@ int main(int argc, char *argv[])
 {
     KLocalizedString::setApplicationDomain("gwenview");
     QApplication app(argc, argv);
+#if QT_VERSION < QT_VERSION_CHECK(6, 0, 0)
     app.setAttribute(Qt::AA_UseHighDpiPixmaps, true);
-
+#endif
     QScopedPointer<KAboutData> aboutData(Gwenview::createAboutData(QStringLiteral("gwenview_importer"), /* component name */
                                                                    i18n("Gwenview Importer") /* programName */
                                                                    ));