The original intent behind not keeping non-modal dialog group transients
was to let the search window in KMail go behind the main window. See bug 76026.
But the proposed solution has some problems:
* first, we're working around "bugs" in KMail. The search window should
not be a transient if one wants to put it behind the main window;
* we're not following the EWMH spec: "Windows that are transient for another
window should be kept above this window."
Even though the spec doesn't explicitly mention group transients, I
think that statement applies to them too.