With plasma 5.7 and qt 5.7 I see weird init problems where only one of the screens gets a background (containment?). So, it has no image, nor a context menu, nor anything. It does however have a panel...
I'll attach a screenshot exhibiting the problem. This is with kscreen having been set to render DVI-I-1 right hand side as primary and HDMI-0 left hand side as !primary.

I'll also attach an xession-errors from a new user which upon first login had the same issue. Second login seemed to initialize the wallpaper fine though, so this might be either racey or only first-config behavior.

Reproducible: Always