Slashback Software

procedure WMNCHitTest(var Message: TWMNCHitTest); message WM_NCHITTEST;
...
procedure TForm2.WMNCHitTest(var Message: TWMNCHitTest);
const
  EDGEDETECT = 7;  //adjust to suit yourself
var
  deltaRect: TRect;  //not really used as a rect, just a convenient structure
begin
  inherited;
  if BorderStyle = bsNone then
    with Message, deltaRect do begin
      Left := XPos - BoundsRect.Left;
      Right := BoundsRect.Right - XPos;
      Top := YPos - BoundsRect.Top;
      Bottom := BoundsRect.Bottom - YPos;
      if (Top<EDGEDETECT)and(Left<EDGEDETECT) then
        Result := HTTOPLEFT
      else if (Top<EDGEDETECT)and(Right<EDGEDETECT) then
        Result := HTTOPRIGHT
      else if (Bottom<EDGEDETECT)and(Left<EDGEDETECT) then
        Result := HTBOTTOMLEFT
      else if (Bottom<EDGEDETECT)and(Right<EDGEDETECT) then
        Result := HTBOTTOMRIGHT
      else if (Top<EDGEDETECT) then
        Result := HTTOP
      else if (Left<EDGEDETECT) then
        Result := HTLEFT
      else if (Bottom<EDGEDETECT) then
        Result := HTBOTTOM
      else if (Right<EDGEDETECT) then
        Result := HTRIGHT
    end;  //with Message, deltaRect; if BorderStyle = bsNone
end;

That's it!  Now Windows will take care of the resizing arrows and the resizing as you drag the edges.