The issue can be reproduced by building a basic GUI application:

1. Create a Qt Widget Application in Qt Creator.
2. Select either QDialog, QWidget or QMainWindow as base class.
3. Set the minimumSize and maximumSize to the same size.
4. Compile & run the application.
5. The application window has no response when clicking the "X" button on the top right corner.

Or, just use designer can observe this issue also:

1. Create a dialog, widget or mainwindow
2. Set the minimumSize and maximumSize to the same size.
3. Select "Preview" from "Form" menu.
4. The preview window has no response when clicking the "X" button on the top right corner.

PS: To close such a window, right click on the title bar and select "Close" from menu.