I'm no Tk expert and don't know any better way, but I found this in the original TclTk documentation

RESIZING PANES

When a pane is resized from outside (e.g. it is packed to expand and fill, and the containing toplevel is resized), space is added to the final (rightmost or bottommost) pane in the window.

So I tried to change the order of panes in a way that your "world" pane A was at the lower right and resizing worked as intended.

  +-----------+ 
  |     C     | 
  +---+-------+
  |   |       |
  | B |   A   |
  |   |       |
  +---+-------+
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