How to install on Linux

First you must configure Proton if you haven't already. After configuring it, it will automatically install itself onto Alien Swarm.

Do this through the following. First press the little gear icon and navigate onto properties (as shown below)



After that navigate to the Compatibility tab of the new window that shows up. Select the "Force the use of a specific Steam Play compatibility tool" checkbox and make sure Proton Experimental is selected in the dropdown below it.

It should look like this once you are finished.

If you launch Alien Swarm now, you will see it being cut off by a black area. Worry not! There is a method to fix this

In the Properties window, go to the General tab. In the Launch Options text box, copy and paste this.

It will start in Windowed Borderless mode, at HD resolution with an FPS cap of 100.

You can change these options if you want, but it must start in Windowed Borderless mode for it to display properly.

Be wary about increasing the cap.
Over 150FPS can lead to laggier server hosting and gameplay. I am unsure if this bug is present in Reactive Drop.

Thanks to Valve Developer Wiki for useful command line option information and Grocery for testing it on KDE Ubuntu. Also thanks to a commenter for testing on LMDE & Artix

Tested by me on Fedora 40.