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When trying to install games, I know that POL download the required POL functions (ie: Steam), then run these locally.

I found the files, however each time I modify a function (in its file), POL re-download the function file, this make impossible for me to test my changes. How to do ?

Hmm, maybe go offline while you test your function.  That may defeat the point when using Steam, but I know Steam works really well offline. 

I actually run a whole LAN party Offline with Steam so I don't need 6 Steam accounts.  Some games like The Ship, Terraria, Half-Life 2 Deathmatch and Counter-Strike: Source all run well offline.

I even setup a second network connection in Mint so I can easily switch between internet and offline (10.0.0.1)

What exactly are you trying to do?

Booman, I wish to try to repair the Steam script (and perhaps other POL functions), that seems currenly break.

Oh, very nice!

Is this related to the new GUI Valve has rolled out and now we can't see anything in the Library or Store?

You may have to contact Valve support about fixing it.  I'm not sure how you can run Steam without it updating upon launch.  If offline doesn't work, then you are stuck.