It's enabled by default in qt6

src/main.cpp

@@ -27,9 +27,10 @@ int main(int argc, char **argv)
 #ifdef HAVE_FACILE
    caml_startup(argv);
 #endif
-
+#if QT_VERSION < QT_VERSION_CHECK(6, 0, 0)
     QCoreApplication::setAttribute(Qt::AA_UseHighDpiPixmaps, true);
     QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
+#endif
     QApplication app(argc, argv);
     KLocalizedString::setApplicationDomain("kalzium");