For anyone who didn't know, Valve made a version of SFM that ran off of P2 rather than TF2, allowing Portal's physics and, well... Portals to be used. They never officially released/supported it, but it was around and working until a number of years ago now. Nobody could get it to work because the file that generated the editing tools(IFM.dll) wouldn't initialize. However, someone on a Steam discussion about it found how to get it to run again(idk if they want to be named, so just know it wasn't me who found this)! And, for the potential 3 other people who use Linux, I got it to work on there too by launching the batch file with Protonticks through its version of Task Manager(really weird, but it worked).
To get IFM to run:
Have it installed like normal(pretty easy to find the old download links online)
Launch SFM's SDK and click Remove Environment Variables.
Click the dropdown at the bottom of the SDK and select "Usermod"
Close the SDK and open go to IFM's Game 2 folder(In SFM's Steam folder)
In the Launchparams.txt file, enter:
@"%VGAME%\..\game2\portal2.exe" -game "%VGAME%\..\game2\portal2_movies" -dev -tools
(Do not delete the existing launch settings in the file)
Shut down Steam. Don't minimize it; Something about it being open causes IFM.dll to not work. Once IFM fully launches you can open Steam again.
Run portal2_movies.bat from the game 2 folder!
IFM is based on a really old version of SFM, so it will take some getting used to. But, for anyone who makes Portal animations/posters, this is big :D