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The method for setting the Window Manager and Session Manager has changed in Leap 15; instead of modifying /etc/sysconfig directly, one now has to use the update-alternatives command or the YaST Alternatives module as described in the Leap 15 Release Notes, part 4.3 (Use update-alternatives to Set Login Manager and Desktop Session).
BTW, what, if any, is the proper KDE3 value to use for the session manager? The last time I set this it was kwin, but the Release Notes are talking about a *.desktop file. (Strangely, for the X windows server, in a directory that doesn't exist.)