When opening a QFrame with Qt::Popup you cannot Alt+Tab into another app

(Reproduced on Ubuntu 23.10)

Steps to reproduce:

• Compile and run the following snippet with Qt 6.6.1:

#include <QApplication>
#include <QFrame>
int main(int argc, char *argv[])
{     
  QApplication a(argc, argv);     
  QFrame f(nullptr, Qt::Popup);     
  f.show();     
  return a.exec(); 
} 

• Try pressing Alt+Tab to switch applications

Expected:

• Application switcher is shown when pressing Alt+Tab

Actual:

• Nothing happens