App window did not appear but was outlined in Mission Control

Clicking in the dock icon in order to making app raise didn't cause it to appear as usual. Nevertheless:

• the outline could be found at Mission Control on mouse over
• sounds and logs have continued to work
• no message from Qt could be found in logs

The symptom can be achieved by setting Window opacity = 0 in a Empty Quick Application

It had happen only once and I was unable to reproduce.