I have a bash script added as a non-steam game, which ensures the game launches in borderless windowed mode.

In order to get rid of the white border around the game, you must first launch the game in borderless windowed mode, and then switch to fullscreen mode after the game has launched.

However, if you forget to change this back, the game will fail to launch next time as it's still set to use fullscreen. You can get around this by using a bash script to replace the config file before the game launches, which will make it launch in borderless windowed mode.

I use the following script, which I've added as a non-steam game. I also have a copy of the called file called user_settings.config.pre which is used to replace the current config:

#!/bin/bash

STEAMDIR="/home/xero/.var/app/com.valvesoftware.Steam/data/Steam/steamapps/compatdata/553850/pfx/drive_c/users/steamuser/AppData/Roaming/Arrowhead/Helldivers2"

if [[ $PWD = $STEAMDIR ]]; then rm user_settings.config fi

cp user_settings.config.pre user_settings.config

sleep 3

xdg-open steam://rungameid/553850