I’ve tried that ─ it’s available from the AUR ─ and on my machine, the result is even worse. It only shows a global menu when both it and the official global menu widget are on the panel, and then you get two global menus. If you remove the official global menu widget and only use the replacement, then the menu is shown inside the application window.
All in all, the bug in the official global menu widget is not that catastrophic. You only have to switch over to a running Qt-based application and back, and then it’ll work for all GTK applications that support it. Not all applications do ─ Pamac and TimeShift do not, for instance. And for Firefox you need to use one of the alternatives instead, as I’ve explained in this thread below. 