ColumnSelection::findColumnPosition returns wrong values for hidden columns

findColumnPosition converts absolute column indices to visible column indices. When given the index of a hidden column it should return the index of the left-most visible column. However
(1) if the hidden column is the leftmost or rightmost column in a hidden region it treats the column as if it is visible
(2) if the hidden column is further inside the hidden region, the calculation of the leftmost visible column position returns incorrect results.