MSG-TV-SELCHANGING
This event is fired when the selected item in a Tree-View control is going to be changed. The EVENT-DATA-2 data item contains the ID of the candidate item. EVENT-DATA-1 contains one of the following symbolic values, defined in 
isgui.def, depending on the reason of the change.
TVFLAG-MOUSE  | The new item is going to be selected with the mouse.  | 
TVFLAG-KEYBOARD  | The new item is going to be selected with the keyboard.  | 
TVFLAG-PROGRAM  | The new item is going to be programmatically selected by setting the  Value property.  | 
Setting EVENT-ACTION to EVENT-ACTION-FAIL prevents the new item from being selected.