Solution to copy & paste on the Terminal
flatpak override --user --env=PROTON_NO_ESYNC=1 com.valvesoftware.Steam
I tried to play World of Warships on Clear Linux using the flatpak of Steam, but the game never started. So, I ran
flatpak run com.valvesoftware.Steam on the Terminal to see if there was any useful information. The error that called my attention was
eventfd: Too many open files. I did a google search and found some mention about setting
PROTON_NO_ESYNC=1 as an environment variable. So, I ran
flatpak override --user --env=PROTON_NO_ESYNC=1 com.valvesoftware.Steam on the Terminal, to set the environment variable
com.valvesoftware.Steam. I tried again and it worked.