Unable to scroll list of sessions by mouse wheel or keyboard

[Norbert]

Steps to reproduce:
1. Have Ubuntu 20.04 LTS installed with many desktop sessions
2. Boot the machine
3. Try to specify graphical user session on the LightDM greeter screen by mouse or by keyboard

Expected results:
* the list of graphical sessions may be scrolled by mouse and/or by keyboard

Actual results:
* it is impossible to view all graphical sessions and scroll to one of them using keyboard and/or mouse (on the attached screenshot it is impossible to view sessions below Dwm).

ProblemType: Bug
DistroRelease: Ubuntu Kylin 20.04
Package: lightdm 1.30.0-0ubuntu3.1 [origin: Ubuntu]
ProcVersionSignature: Ubuntu 5.4.0-28.32-lowlatency 5.4.30
Uname: Linux 5.4.0-28-lowlatency x86_64
NonfreeKernelModules: zfs zunicode zavl icp zcommon znvpair
ApportVersion: 2.20.11-0ubuntu27
Architecture: amd64
CasperMD5CheckResult: skip
CurrentDesktop: Deepin
Date: Thu Apr 30 19:48:12 2020
InstallationDate: Installed on 2020-04-22 (7 days ago)
InstallationMedia: Ubuntu-MATE 20.04 LTS "Focal Fossa" - Release amd64 (20200422)
LightdmConfig:
 [Seat:*]
 autologin-guest=false
 allow-guest=false
SourcePackage: lightdm
UpgradeStatus: No upgrade log present (probably fresh install)

[Sebastien Bacher (seb128)]

Thank you for taking the time to report this bug and helping to make Ubuntu better. This particular bug has already been reported and is a duplicate of bug 1279345, so it is being marked as such. Please look at the other bug report to see if there is any missing information that you can provide, or to see if there is a workaround for the bug. Additionally, any further discussion regarding the bug should occur in the other report. Feel free to continue to report any other bugs you may find.