I've got my laptop display (LVDS1) as a "Primary output" and an external monitor (VGA1). Sometimes when I attach the external monitor my open windows are moved there. It's quite annoying - I'd much prefer for them to stay on my laptop screen and only move what I want to move to the external monitor. Toggling the "Primary output" star-icon makes no difference. FWIW the laptop is Lenovo T420s and it happens with a number of different external monitors I use. Package is kscreen-1.0.1-11.1 from OpenSUSE KDE 4.11 testing repository.
Windows are managed by KWin, reassigning
there is not really a bug here. Windows open on the currently active screen. That is the screen with the active application unless you use the option to have active screen follows mouse.
> Sometimes when I attach the external monitor my open windows are moved there. I think what he observes is "bug" 296673 @mludvig The windows are actually not moved, but your screen(s) move below them, ie. the global absolute position (eg. 200+100) is now on another screen. This happens whenever screens are added to the top or the left. *** This bug has been marked as a duplicate of bug 296673 ***