A control sends a WM_PARENTNOTIFY message whenever the user presses a mouse button while pointing to the control. Some applications interpret this message as a signal to carry out an action related to the control, such as displaying a line of text describing the purpose of the control.
The system also sends WM_PARENTNOTIFY messages when it creates and destroys a window, but not for controls created from a dialog box template. The system prevents these messages by specifying the WS_EX_NOPARENTNOTIFY style when creating the controls. An application cannot override this default behavior unless it creates its own controls for the dialog box.