TControl.ScreenToClient
From OpenEUO
ME ScreenToClient(nX,nY) Converts the screen coordinates of a specified point on the screen to client coordinates.
Use ScreenToClient to convert a point in screen coordinates to local, or client area, coordinates. In client area coordinates (0,0) corresponds to the upper left corner of the control's client area. In screen coordinates (0,0) corresponds to the upper left corner of the screen. Use ScreenToClient along with ClientToScreen to convert from one control's coordinate system to another control's coordinate system.
