X11: showing a QMessageBox from within QDialog::accept() makes it lose focus

On X11, I have these:

• A main window with a button. Clicking that button will show a QDialog.
• Override the QDialog::accept() and show a QMessageBox.
• When the message box closes, the dialog loses focus to main window.

I think the dialog should hold its focus when the message box closes.

I'm attaching source code which reproduces that behavior.

Would you please check?

Thank you,