In
QGuiApplicationPrivate::commitData()
the two lines
if (session_manager->allowsInteraction() && !tryCloseAllWindows())
session_manager->cancel();
cause windows to be closed, which results in the application quitting if QGuiApplication::quitOnLastWindowClosed() is true, which is the default.
That is NOT what the SaveYourself message of the X session management protocol is supposed to trigger in applications. See:
http://www.x.org/releases/X11R7.6/doc/libSM/xsmp.html#protocol
Currently, qxcbsessionmanager.cpp is the only call site of QPlatformSessionManager::appCommitData(), which calls QGuiApplication::commitData(), so QGuiApplication::commitData() should implement at least the XSMP spec correctly.

This bug prevents some KDE applications from restoring correctly.