After the QML suddenly failed, I reloaded the complete Qt system. It still will not respond to mouse inputs to any button, even the 'x' close button. The C++ interface outputs are not displayed. If the program is started from the debugger it hangs loading the thread in app.exe. This is a large program that has been working. A previous version saved to the desktop now displays the same symptoms.
Linux Mint 20.3, QtCreator 11.0.2, Qt 6.4.3
Tried deleting and reinstalling the entire Qt platform. This made no difference.