In Firefox, logged in at https://forums.virtualbox.org/posting.php?mode=reply&t=113890 whlist drafting a reply, I clicked to add an image. [img][/img] code appeared, so I keyed Control-T for a new tab then went to my logged in view of https://imgbb.com/.
Then probably Alt-Tab to bring Dolphin to the foreground, and whilst I began browsing screenshots, if I recall correctly:
- on my secondary display, an HP ZBook 17 G2 (docked), the screen became distorted
- apps disappeared from my main display, to the left (on DisplayPort)
- some apps reappeared
- plasmashell reappeared