Just have look at this screenshot and guess what makes it differ from the developer mode viewport windows in the currently released beta 18.
Nothing? Way wrong: The viewport window is now a full GTK+ featured window instead of just a raw X window. Currently, this makes no difference indeed, but it offers quite interesting possibilities: We could easily add scrollbars to the window, just like on Windows. This not only enables drawing large landscapes without having to drag a player's clonk around to move the viewport's position but due to the main loop still running during scrolling, it would also even pretend the game from blocking (like the windows version does) which is quite annoying when drawing landscapes with others through the network. Well, of course the network first needs to be fixed to stay synchronized with the windows version.
Another possibility the GTK+ window offers is drag+drop of definitions from, say, nautilus, into the viewport with few lines of code and having GTK+ do the dirty work.