Ttk's panedwindow widget.
MHo 2008-04-09: Does someone know if and how it's possible to determine the dimension of a pane inside a panedwindow? I have a canvas inside a pane which holds an image. If resizing / maximizing the toplevel windows, everything should grow to use the extra space. But the paine itself says it is has the dimension 1x1, and the canvas stays as its initial size (if I don't specify such initial size, something else goes wrong which is beyond the scope of this question....)