Das wird doch gefeuert wenn man den Fokus vom Fenster verliert und wieder bekommt, oder? Bin mir da selbst gerade nicht sicher.
//edit
Zitat:
Use OnActivate to perform special processing when the form receives focus. A form becomes active when focus is transferred to it (when the user clicks on the form, for example).
Note: The OnActivate event of the application (TApplication), not the form, occurs when Windows switches control from another application.