What you're trying to do is likely to damage your windows steam installation. Don't do that. That's not how it works.
As mentioned you cannot share installations between OS versions. Leaving this open as a feature request but this is unlikely to change.
Yet with Proton i would really need something to force to use the Windows Version of a Game (like for example PAYDAY 2)
Nothing extracted yet.
More explanations:
I have a windows partition mounted at boot (fstab) with steam installed and way much more space that in my linux partition.
I want to use the already downloaded windows-side steam games files (system independant files like maps, voices, models, etc.) so it would not take extra space in my hard drive.
But I run into the "new steam folder must be empty"
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