mpg uses the Tk windowing/graphics library. There are two implementations of Tk for Mac OS X. One, Tk-X11, is built on top of the X11 window system. This version of Tk is very similar to Tk on other platforms, including GNU/Linux, on which this program has been developed. If you want the program to work as closely as possible to its design, you should install X11 and Tk-X11.
Tk-Aqua is built on top of Aqua, the native windowing/graphics system for Mac OS X. If you use Tk-Aqua, you will get a look-and-feel that is closer to that of other native Macintosh programs than if you use Tk-X11. The downside is that, by enforcing a numver of user interface design decisions made by Apple, Tk-Aqua cannot do some things that Tk-X11 can. Among the differences are the following:
(a) Tk-Aqua will only put menus on the menubar. It refuses to put
command buttons on the menubar.
(b) Tk-Aqua does not support multi-column menus.
(c) Tk-Aqua does not support multi-line button labels.
The most important of these is the first. This program makes use of commands on the menubar. If it is run unmodified under Tk-Aqua, these commands are simply omitted. In order to allow the program to be used under Tk-Aqua, if it detects that it is running under Tk-Aqua, it creates a menubar of its own at the top of the window containing buttons for these commands. The other menubar entries are put on the usual Mac menu at the top of the screen. This makes the commands accessible at the expense of splitting the menu into two parts.