This one is not a leak per se.
It is a globally acquired resorce that does not get freed.
It does not have to be freed, as -when the program terminates- it is freed automatically.
Therefore it is a bit more efficient not to free it explicitly. Causes you a bit of pain when debugging with valgrind.

As a rule of thumb, Qt itself is generally free of memleaks as the Trollies use valgrind extensively.
Unless the leaks accumulate it is usually best to ignore Qt/X11 'leaks' and concentrate on your own. valgrind tells you (backtrace) if the leak is one of yours.

HTH