Captures the mouse and tracks its movement until the user releases the left button, presses the ESC key, or moves the mouse outside the drag rectangle around the specified point.
Initial position of the mouse, in screen coordinates. The function determines the coordinates of the drag rectangle by using this point.
If the user moved the mouse outside of the drag rectangle while holding down the left button , the return value is nonzero.
If the user did not move the mouse outside of the drag rectangle while holding down the left button , the return value is zero.
This member function emulates the functionality of the function DragDetect, as described in the Windows SDK.