You can use this event to customize the automatic sizing behavior that occurs when a user double-clicks a column divider. For example, you can handle this event to programmatically resize columns based only on the content in the rows that are currently visible, avoiding a performance impact with the default behavior when working with a large number of rows.
The column DataGridViewColumn.DividerWidth property does not extend the area in which a user can double-click. If the width of a divider has been increased through the use of this property, this event occurs when the user double-clicks the boundary between the divider and the adjacent column.
For more information about handling events, see Consuming Events.