top: correct an obscure column highlighting buglet

top: correct an obscure column highlighting buglet

An anomaly would surface whenever the sort column was
made non-display under fields management.

The scroll message would report incorrect x coordinates
and the 'x' toggle would produce an unintentional
horizontal scroll.

This patch forces column highlighting off whenever field
changes are made (as is done for horizontal scrolling).
It also inactivates the 'x' toggle whenever the sort
column is not visible.

This bug has yet to be reported.