desktop shows last active window instead of widgets

I'm using Kubuntu 15.04 with latest plasma-desktop 4:5.2.2-0ubuntu4 version, with sddm.
When I look at my desktop background, I get the attached screenshot. This is the first time this happens, last logins were ok (exposed other bugs though).
The content of the background changes, depending on which window has been highlighted before. When I have firefox and hexchat open, and switch from hexchat to firefox, the desktop content changes to hexchat.