My instalation:
Ubuntu 18.04
Horizon-Client-5.2.0-14604769.x64
rdesktop 1.8.6 --disable-smartcard --disable-credss
Windows 2008R2
I have i problem. if we use ESC on remote VM - we exit FullScreen mode and can't back. I have check this issue on Horizon 4,6 - 5.2.