TForm1 = class(TCustomForm)
private
  const
    WM_AFTERACTIVATE = WM_USER + 1;
  var
    FInitialized: Boolean;
private
  procedure WMAfterActivate(var Message: TWMNoParams); message WM_AFTERACTIVATE;
end;

procedure TForm1.FormCreate(Sender: TObject);
begin
  FInitialized := False;
end;

procedure TForm1.FormActivate(Sender: TObject);
begin
  if not FInitialized then
  begin
    FInitialized := True;
    if not PostMessage(Handle, WM_AFTERACTIVATE, 0, 0) then
      RaiseLastOSError();
  end;
end;

procedure TForm1.WMAfterActivate(var Message: TWMNoParams);
begin
  ....
end;