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HDN_BEGINTRACK

HDN_BEGINTRACK 
    phdn = (LPNMHEADER) lParam; 

Notifies a header control's parent window that the user has begun dragging a divider in the control (that is, the user has pressed the left mouse button while the mouse cursor is on a divider in the header control). This notification message is sent in the form of a WM_NOTIFY message.

• Returns FALSE to allow tracking of the divider, or TRUE to prevent tracking.

phdn

Address of an NMHEADER structure that contains information about the header control and the item whose divider is to be dragged.

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