Comment # 2 on bug 1185530 from Fabian Vogt

(In reply to Felix Miata from comment #0)
> Created attachment 848942 [details]
> screenshot of Desktop Effects window
> 
> Original summary:
> scrollbar is absent in systemsettings5 > workspace behavior > desktop effects
> 
> This is on 15.3 RC1 freshly installed en_US only in KDE role without
> recommends, with plastik window decorations.

You need qqc2-desktop-style, which is only installed with weak deps enabled.
Without it, the "Universal" style is used which has overlay scrollbars. It
appears when scrolling or hovering the right edge.
It's only a weak dep because it still works fine without having it installed.

This is at least the third time this was reported as a bug though, so adding it
has hard dep of the kde_plasma pattern appears to be necessary now...