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private
    procedure onReMaximize; 
end;

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var
  LastRezizeWindowState : Cardinal;

implementation

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procedure TForm1.FormResize(Sender: TObject);
var wp : TWindowPlacement; 
begin

  wp.length := SizeOf (wp);
  GetWindowPlacement (Handle, @wp);  

  if LastRezizeWindowState = SW_SHOWMAXIMIZED then begin
    if wp.showCmd=SW_SHOWNORMAL then begin
      Form1.WindowState:=wsNormal;

      onReMaximize;

    end;
  end;

  LastRezizeWindowState:=wp.showCmd;
  
end;					

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//...
  public
    procedure WMSize(var Msg: TWMSize); message WM_SIZE;
//...
procedure TForm1.WMSize(var Msg: TWMSize);
begin
  if Msg.SizeType = SIZE_RESTORED then
    if Tag = SIZE_MAXIMIZED then
      showmessage('Hallo');
  Tag := Msg.SizeType;
end;					

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inherited;