To reproduce:
- Add ColumnGroupsViewDefinition and subscribe to ColumnWidthChanged event.
- Start the application and resize a column.

Workaround:
public class MyColumnGroupRowLayout : ColumnGroupRowLayout
{
    public MyColumnGroupRowLayout(ColumnGroupsViewDefinition view) : base(view)
    {
    }

    public override void ResizeColumn(int delta)
    {
        base.ResizeColumn(delta);
        //This method will be called when a column is resized
    }
}

class MyColumnGroupsViewDefinition : ColumnGroupsViewDefinition
{
    public override IGridRowLayout CreateRowLayout()
    {
        return new MyColumnGroupRowLayout(this);
    }
}