The number of visible leading columns locked in a browse widget. If a locked column is hidden, the next visible non-locked column in the browse will then become locked.
When you use the horizontal scrollbar to scroll columns in the browse, locked columns do not move. For example, if NUM-LOCKED-COLUMNS is 3, then the three leftmost columns in the browse are locked.
Note: In character mode, this attribute can only be set before the widget is realized.
