Version: 6.8.x and 2003.x
The command SET LIST PROPERTIES allows you to define the expandable/collapsable attribute of the sublist when the parent list is double-clicked. However, this property can only be set if the appearance, icon and lineHeight are passed to the command as well. The following is the syntax requirement:

SET LIST PROPERTIES (list; appearance{; icon{; lineHeight{; doubleClick}}})

Here is a generic method that will set the expandable and collapsable on double-click property with the minimal number of parameters.

 ` Project Method: Set_ExpColOnDBClick
 ` $1 - List Reference
 ` $2 - Expand/Collapse sublist on double-click
 `   True = Yes, False = No
C_LONGINT($1;$appearance;$icon;$lineHeight;$doubleClick)
C_BOOLEAN($2)
If (Is a list($1))
     GET LIST PROPERTIES($1;$appearance;$icon;$lineHeight;$doubleClick)
    If ($2)
      SET LIST PROPERTIES($1;$appearance;$icon;$lineHeight;0)
    Else
      SET LIST PROPERTIES($1;$appearance;$icon;$lineHeight;1)
    End if
End if

A call to that method would look like this: Set_ExpColOnDBClick($ListRef;$ExpandCollapse). If $ExpandCollapse is set to True, the list will expand or collapse when double-clicked.