My laptop is connected to an external monitor, which is the primary display. Clicking the `Widget style` combo box opens it on the laptop display. Reproducible: Always Steps to Reproduce: 1. Run Kubuntu 16.04 from a live USB stick 2. Open the Widget Style and Behavior dialog 3. Click on "Breeze".
Created attachment 100655 [details] Combo box drop-down opens away from source
This looks like https://bugreports.qt.io/browse/QTBUG-53813 It's not a KWin bug as such windows are so called override-redirect windows. They are not managed by a window manager, but position themselves.