User blog comment:Otakon7/Otakon7's Buddyfight Simulator/@comment-10858287-20170912200253/@comment-2234004-20170914155027

It's actually the server settings (sadly above me, the host has default setings), if you're inactive for a while, it clears your session.

One way around it is to have the main menu open in a new tab and periodically refresh that tab, and if it happens to log you out, you just login in again in that tab and your deck builder will still work.