I started running into the issue about a year ago when I switched to a multi-monitor setup and discovered that every boot I had to reconfigure the monitors. I did some searching and discovered that it was a known KDE problem with no documented fix.
It doesn't require you to use KDM, it's a problem in the KDE screen management after KDE starts.
Posted May 1, 2012 19:56 UTC (Tue) by mathstuf (subscriber, #69389)
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I've noticed that KDE's RandR configuration tool is…oddly behaved at times (even recently when helping a coworker set up 3 monitors). The command line is much better for RandR configuration (I've found no GUI that is faster to get what I want or more convenient) and I have a script in ~/.config/xinit which gets sourced to set up rotation and positioning upon login (no modeline commands though, just relative placement).