Получить первую или последнюю видимую строку в TRichEdit

function RE_GetLastVisibleLine(RichEdit: TRichEdit): Integer;
 const
   EM_EXLINEFROMCHAR = WM_USER + 54;
 var
   r: TRect;
   i: Integer;
 begin
   { 
   The EM_GETRECT message retrieves the formatting rectangle 
   of an edit control. 
  }
   RichEdit.Perform(EM_GETRECT, 0, Longint(@r));
   r.Left := r.Left + 1;
   r.Top  := r.Bottom - 2;
   { 
    The EM_CHARFROMPOS message retrieves information about the character 
    closest to a specified point in the client area of an edit control 
  }
   i := RichEdit.Perform(EM_CHARFROMPOS, 0, Integer(@r.topleft));
   { 
    The EM_EXLINEFROMCHAR message determines which 
    line contains the specified character in a rich edit control 
  }
   Result := RichEdit.Perform(EM_EXLINEFROMCHAR, 0, i);
 end;
 
 { 
  Sending the EM_GETFIRSTVISIBLELINE message to a multi-line edit control 
  finds out which line is the first line visible. 
  This is the line that is currently displayed at the top of the control. 
}
 
 function RE_GetFirstVisibleLine(RichEdit: TRichEdit): Integer;
 begin
   Result := RichEdit.Perform(EM_GETFIRSTVISIBLELINE, 0, 0);
 end;
 

Взято с сайта: https://www.swissdelphicenter.ch