The problem is it needs to get fixed width and that is not fixable by CSS only. You need to get the width from the table-responsive container and apply it to the tablesorter headers wrapper, which becomes position: fixed (and that is why it expands). Another peace of JS would be needed when you have very wide table going behind the edge and when you scroll it horizontally, the tablesorter column headers need to scroll simultaneously. Not a trivial fix IMHO.
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