WM_INITMENUPOPUP    
The
WM_INITMENUPOPUP message is sent when a drop-down menu or submenu is about to
become active. This allows an application to modify the menu before it is
displayed, without changing the entire menu. 
WM_INITMENUPOPUP 
hmenuPopup = (HMENU) wParam;         // handle of submenu
uPos = (UINT) LOWORD(lParam);        // submenu item position 
fSystemMenu = (BOOL) HIWORD(lParam); // window menu
flag 
 
Parameters
hmenuPopup
Value of wParam.
Identifies the drop-down menu or submenu. 
uPos
Value of the
low-order word of lParam. Specifies the zero-based relative position of
the menu item that opens the drop-down menu or submenu. 
fSystemMenu
Value of the
high-order word of lParam. Specifies whether the drop-down menu is the window
menu (also known as the System menu or the Control menu). If the menu is the window
menu, this parameter is TRUE; otherwise, it is FALSE. 
Return Values
If an
application processes this message, it should return zero. 
See Also