fix build errors on Linux with Clang 20 and GTK warnings (#12507)

* guard -Wno-error=enum-constexpr-conversion behind compiler flag check

* Fix: Suppress GTK critical warnings and prevent multiple URI scheme registrations in WebView

src/slic3r/GUI/GUI_App.cpp

@@ -2697,6 +2697,18 @@ bool GUI_App::on_init_inner()
     g_object_set (gtk_settings_get_default (), "gtk-menu-images", TRUE, NULL);
 #endif
 
+#if defined(__WXGTK20__) || defined(__WXGTK3__)
+    // Suppress harmless GTK critical warnings from the GTK3/wxWidgets interaction.
+    // These include widget allocation on hidden widgets and events on unrealized widgets.
+    g_log_set_handler("Gtk", G_LOG_LEVEL_CRITICAL,
+        [](const gchar *log_domain, GLogLevelFlags log_level, const gchar *message, gpointer user_data) {
+            if (message && (strstr(message, "gtk_widget_set_allocation") ||
+                            strstr(message, "WIDGET_REALIZED_FOR_EVENT")))
+                return;
+            g_log_default_handler(log_domain, log_level, message, user_data);
+        }, nullptr);
+#endif
+
 #ifdef WIN32
     //BBS set crash log folder
     CBaseException::set_log_folder(data_dir());