I can confirm this bug. I was about to file it myself. I have an ATI Radeon X300 using the foss radeon driver. I have dual monitors set up using xrandr.
Using kwin 3.5.9.dfsg.1-6 (stable) my setup works perfectly. When I upgraded to 3.5.10.dfsg.1-2 (testing), kwin maximizes windows across both screens, which is very annoying. It seems to have lost the idea that my monitors are separate, despite the fact that in the "configure desktop" -> display -> multiple monitors dialog, I correctly had the two screens divided and kwin should be obeying these settings. This is despite KDE itself knowing I have two separate monitors -- they both show up separately in that dialog, they identify properly when I hit "identify screens", and backgrounds are properly broken across them rather than stretching. It's very annoying, to say the least -- I had to downgrade! Hope this can be fixed. -- To UNSUBSCRIBE, email to debian-bugs-dist-requ...@lists.debian.org with a subject of "unsubscribe". Trouble? Contact listmas...@lists.debian.org