Adjust columns in new mail layout

Adjust columns in new mail layout

The old Zoho mail layout allowed us to easily drag the column widths to make our work more effective. I'm now working with the "new" mail layout, and this functionality seems to have been removed. When I mouse over the edges of the columns nothing happens.

Is this possible somehow and I'm doing it wrong? If not possible, why in the world would you take out what is very basic functionality for adjusting the column widths? Thanks.