If you post a minimal working example, maybe someone will see a way. My first thought is to somehow bind to the window resize, and then do some adjustments. Or maybe put your sort buttons in a separate frame above the spreadsheet and try to figure out how to align the buttons with the columns.

$mw->bind( '<Configure>', sub{ &OnResize });
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Or switch to Gtk2, which has a good table widget support, where you can individually set the expand,shrink,and fill setting for each table element.