What were you trying to do that didn't work?

When we implement restricting the session to a single application, specify in Chapter 21.
Firefox doesn't start in fullscreen, there is a part of the display black.

What is the impact of this issue to you?

It's not usable for end users.

Please provide the package NVR for which the bug is seen:

How reproducible is this bug?:

Steps to reproduce

1. yum install gdm firefox

1. useradd -Z user_u kioskuser
2. touch /usr/share/xsessions/kiosk.desktop
3. add following content in /usr/share/xsessions/kiosk.desktop
[Desktop Entry]
Name=Kiosk
Type=Application
Exec=firefox www.ameli.fr
DesktopNames=kiosk

4. touch /var/lib/AccountsService/users/kioskuser
5. write following content in /var/lib/AccountsService/users/kioskuser
[User]
Session=kiosk
SystemAccount=false
6. Add following content in /etc/gdm/custom.conf in [daemon] section
AutomaticLoginEnable=True
AutomaticLogin=kioskuser
TimedLoginEnable=True
TimedLogin=kioskcpam
TimedLoginDelay=0
7. Add following content in /etc/security/namespace.conf
/tmp tmpfs tmpfs ~kioskuser
/var/tmp tmpfs tmpfs ~kioskuser
$HOME tmpfs tmpfs ~kioskuser
8. Enable SELinux polyinstantiation
setsebool -P polyinstantiation_enabled on
9. Add following content in /etc/systemd/logind.conf in [login] section
UserStopDelaySec=0

Expected results

Firefox ephemeral in fullscreen

Actual results

Firefox is not in fullscreen, we have a large black band at right and left