When you lock ubuntu with keyboard or mouse with the right top menu, the sidebar stays still. The applications can be run from there. No input we can provide for these apps, but still we can use every feature the sidebar provides including starting "Compose new message" window.
The attachment is the screenshot of related to the problem. PrintScreen button worked and took screenshot of screen, saved it when we are not logged in. Spotify example is one such example shows the problem. ( Screenshot from 2019-02-26 03-30-54.png, Screenshot from 2019-02-26 03-38-58.png).
Have a good day.
ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: evince 3.28.4-0ubuntu1
ProcVersionSignature: Ubuntu 4.15.0-45.48-generic 4.15.18
Uname: Linux 4.15.0-45-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.5
Architecture: amd64
CurrentDesktop: ubuntu:GNOME
Date: Tue Feb 26 03:43:04 2019
InstallationDate: Installed on 2018-10-29 (119 days ago)
InstallationMedia: Ubuntu 16.04.5 LTS "Xenial Xerus" - Release amd64 (20180731)
SourcePackage: evince
UpgradeStatus: Upgraded to bionic on 2018-10-29 (119 days ago)
This bug was fixed in updates a few weeks ago (bug 1769383).
What version of package 'gnome-
Also, please run this command to send us more information about the system:
apport-collect 1817646