• Create a QWidget project in QtCreator
• Throw a QLineEdit or a QTextEdit into the main .ui file
• Run and press Command + w

==========> The window will not close and a 'w' will appear in the edit box.

It seems that Qt checks for a series of QKeySequences during a key handling loop. If the key combination is none of those QKeySequences, it is just interpreted as a key press. In Qt4, the window would have closed.