Timberborn runs fine on Proton, with the one exception that alt-tab causes the game to stop updating the screen. It keeps running (and you can click on stuff) but you're left with either a black window or a frozen window.
You can run the game in a window to prevent this from happening, but then you can't use your full resolution (or have to accept not seeing the bottom of the screen). On some Linux WMs, you can just tell it to run full-screen anyway while the game thinks it is in windowed mode, but on most compositing WMs this doesn't work.
On Windows, there's an open source app called Borderless Gaming to resolve this problem. It is for sale on Steam, though I have no idea if it can be made to run on Proton in combination with another game, given that they don't share the same Wine prefix. However, you can download it for free from the releases page here: https://github.com/Codeusa/Borderless-Gaming/releases/tag/9.5.6
# Step 1: install Borderless Gaming into the game's Wine prefix:
cd $HOME/.steam/steam/steamapps/compatdata/1062090 # Timberborn's Steam ID; swap for whatever game
export STEAM_COMPAT_CLIENT_INSTALL_PATH="$HOME/.steam/steam"
export STEAM_COMPAT_DATA_PATH="$HOME/.steam/steam/steamapps/compatdata/1062090"
export WINEPREFIX=$PWD
# Adjust the next two for the version of Proton you're running and where you downloaded BorderlessGaming's installer
~/.steam/root/steamapps/common/Proton\ -\ Experimental/proton run $HOME/Downloads/BorderlessGaming9.5.6_admin_setup.exe
# Don't run BG yet; it will stop most games from launching.
# Launch the game from Steam, configure it to be windowed but at your full resolution, then in the same window you did the exports above:
~/.steam/root/steamapps/common/Proton\ -\ Experimental/proton run ./drive_c/Program\ Files\ \(x86\)/Borderless\ Gaming/BorderlessGaming.exe