How to adjust width of "Corners" column (the 1st column) in Maestro Corner Setup window?

Hello,

I am using Virtuoso Custom IC Design Environment version IC6.1.8-64b.500.27.

In Corner Setup window the first column's width adjustment vertical line segment is grayed out (as can be seen in the inserted snapshot). Appreciate any suggestion on how I may adjust this column's width. By the way, I do have Virtuoso ADE Assembler open for Edit.

Thank you,
Amit