Owner-Drawn Menus and the WM_MEASUREITEM Message

Before Windows displays an owner-drawn menu item for the first time, it sends the WM_MEASUREITEM message to the window procedure of the window that owns the item's menu. This message contains a pointer to a MEASUREITEMSTRUCT structure that identifies the item and contains the item data that an application may have assigned to it. The window procedure must fill the itemWidth and itemHeight members of the structure before returning from processing the message. Windows uses the information in these members when creating the bounding rectangle in which an application draws the menu item. It also uses the information to detect when the user chooses the item.