Special Aspects for Menus with a Default Button 
For a menu with a default button, the ALV Grid Control triggers the following events:
If the menu arrow is clicked, event menu_button is triggered.
If the default button is clicked, event user_command is triggered.
For both events, the system passes the function code that you assigned in the function field. So if you want to assign a menu function to the default button, the function code for the menu must be identical to that of the menu function. In the menu_button event, you then build the menu, while in the user_command event, you can directly query the function code for the default function.
Note
See also report BCALV_GRID_08 in the Reuse Library.