Fix of the "dock" menu on OSX to start a new instance of PrusaSlicer

in case a single instance is disabled.
Make the "single_instance" enabled by default on OSX initially
to match the platform.

src/slic3r/GUI/MainFrame.cpp

@@ -56,7 +56,9 @@ public:
     PrusaSlicerTaskBarIcon(wxTaskBarIconType iconType = wxTBI_DEFAULT_TYPE) : wxTaskBarIcon(iconType) {}
     wxMenu *CreatePopupMenu() override {
         wxMenu *menu = new wxMenu;
-        if(wxGetApp().app_config->get("single_instance") == "1") {
+        if(wxGetApp().app_config->get("single_instance") == "0") {
+            // Only allow opening a new PrusaSlicer instance on OSX if "single_instance" is disabled, 
+            // as starting new instances would interfere with the locking mechanism of "single_instance" support.
             append_menu_item(menu, wxID_ANY, _L("Open new instance"), _L("Open a new PrusaSlicer instance"),
             [this](wxCommandEvent&) { start_new_slicer(); }, "", nullptr);
         }