fix buildWindow sizing problem [1303]

Hey Jon, let me know how you feel about this one when you get a chance.. I just force the new ViewManager's containing window to do a pack() after the blue "last active" border is added by setLastActive (which doesn't happen until the FRAME_DONE event is received).

It fixes the issue for me and I think it's a general enough solution to the problem.  Just not sure whether it's a good idea to do a pack() there...