unit Unit1;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs;

type
  TForm1 = class(TForm)
  private
    procedure WMSysCommand(var Msg: TWMSysCommand); message WM_SYSCOMMAND;
    procedure WMWindowPosChanged(var Msg: TWMWindowPosChanged); message WM_WINDOWPOSCHANGED;
  public
    procedure CenterForm;
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

procedure TForm1.CenterForm;
var
  R: TRect;
begin
  R := Screen.MonitorFromWindow(Handle).WorkareaRect;
  Top := (R.Bottom - R.Top - Height) div 2;
  if Top < 0 then
    Top := 0;
  Left := (R.Right - R.Left - Width) div 2;
  if Left < 0 then
    Left := 0;
end;

procedure TForm1.WMSysCommand(var Msg: TWMSysCommand);
begin
  inherited;
  if Msg.CmdType = SC_RESTORE then
    CenterForm;
end;

procedure TForm1.WMWindowPosChanged(var Msg: TWMWindowPosChanged);
const
  SWP_STATECHANGED = $8000; // undocumented
begin
  inherited;
  if (Msg.WindowPos.flags and SWP_STATECHANGED <> 0) and (WindowState = wsNormal) then
    CenterForm;
end;

end.