QGuiApplication wrongly tries to ::tryCloseAllWindows() in QGuiApplicationPrivate::commitData()

Hi Qt developers,

KDE developers argued about Session management issue in KDE5 https://bugs.kde.org/show_bug.cgi?id=341930

There are some QTBUG reported already:
QTBUG-46310 integrated by Qt5 v5.5.1
QTBUG-45484 manually applied the patch https://codereview.qt-project.org/#/c/113901/

But I found that ksmserver still missing some clients or windows when storeSession and performLegacySessionSave https://bugs.kde.org/show_bug.cgi?id=354724

Andreas and Thomas argued that QGuiApplicationPrivate::commitData() (supposed to make sure that the session is saved) calls
QApplicationPrivate::tryCloseAllWindows() which, when QGuiApplication::quitOnLastWindowClosed() is true, invokes
QApplication::quit()

it made ksmserver failed to handle session management correctly.

Regards,
Leslie Zhai