system Fedora 25 version xfwm4-4.12.3-3.fc24.x86_64 NVIDIA-Linux-x86_64-375.39 kernel 4.9.9-200.fc25.x86_64 VGA-0 connected primary 1920x1200+0+0 open delete dialogue on dolphin and resolution changes to 640x480 xrandr then does shows 1920x1200 but says unavailable logout reset X and login needed to reset display kwin will only use 640x480 (bug submitted at KDE-376512
xfwm4 (the window manager) has nothing to do with this, this is a KDE issue.