When you suspend the computer the lock screen appears with a fade in. With composition disabled, the fade in comes from a solid background. With composition enabled, the fade in comes from the actual screen contents. This allows you to take a camera, press record, unsuspend the computer and get a photo of what the user had on the screen.
The screen is supposed to be locked before going to suspend. In fact the lock screen ensures that suspending gets delayed till the screen is locked. Could you please provide the output of: qdbus org.kde.KWin /KWin supportInformation
This is the kscreen effect fading the screen to black before turning the screen off / fading back in, screen locker is fine. See https://phabricator.kde.org/D9608
Created attachment 109640 [details] Request KWin support information I attached the KWin support information.
Git commit 437a36a2bb86222f1c3d086c15a2f60eef898c1a by David Edmundson. Committed on 26/04/2018 at 09:15. Pushed by davidedmundson into branch 'Plasma/5.12'. [KScreen Effect] Fix fade to black Summary: When you have two windows ontop of each other and you turn them transparent, evidently you'll be able to see through them. Not only does it look unpolished if the desktop window flashes through your windows, it can pose a privacy risk as you'd be able to briefly look through the lock screen. This patch fades apps to black intead of to transparent. As this looks weird for panels, so windows are also faded to opaque (if relevant) at the same rate. Reviewers: #plasma, graesslin Subscribers: luebking, plasma-devel, kwin, #kwin Tags: #plasma Differential Revision: https://phabricator.kde.org/D9608 M +16 -12 effects/kscreen/kscreen.cpp https://commits.kde.org/kwin/437a36a2bb86222f1c3d086c15a2f60eef898c1a
*** Bug 354008 has been marked as a duplicate of this bug. ***