KDM screen lock & fluxbox: failure to properly lock session

[user11]

I'm using KDM (4.10.2; USE: consolekit handbook pam -aqua -debug -kerberos)
and fluxbox.

I want to lock my screen and allow other users start new session.

So I use kscreenlocker_greet by just running /usr/lib/kde4/libexec/kscreenlocker_greet from my fluxbox menu.

(I am not sure that this is the right command)

The lock message is displayed as desired and the users can start new session as desired.

The problem is that I can press e.g. alt-tab and contol other windows that are on my desktop (without seeing them). I can even press Alt+F2 and execute anything. The command executes without any password. (I don't see that until I unlock screen.) Surely, that is not proper session locking.

The only solution I've found up to the moment is to switch from kde lock to xscreensaver (see https://wiki.archlinux.org/index.php/XScreenSaver#KDM for user switching instruction).

The most similar problem (KDM+fluxbox+multiuser) was found on https://bbs.archlinux.org/viewtopic.php?id=161453 - without solution and without any safety notice. (Maybe irrelevant.)