Hmm. I've been experimenting with this, and it seems wildly inconsistent. Sometimes new windows will appear under existing ones; sometimes they won't. Sometimes they'll appear under other Steam windows but on top of things like Firefox; other times they appear under everything.
We should go check how the focus stealing prevention works on KWin. And also, dialogs should not be able to go behind the main window. This bug may be a duplicate of issue #1040
Focus stealing prevention works with _NET_WM_USER_TIME, which Steam does not provide.
Nothing extracted yet.
I use Kubuntu 12.10 and KDE 4.10. I have focus stealing prevention set to high. Whenever I do an action which creates a new window, it starts in the background and I have to explicitly switch to it. For example, if I right click on a game and click properties, the property window starts in the background.
According to KDE docs these type of windows should be launched already active.
http://docs.kde.org/stable/en/kde-workspace/kcontrol/windowbehaviour/index.html
High: New windows get activated only if no window is currently active or if they belong to the currently active application. This setting is probably not really usable when not using mouse focus policy.