Fava seems to serve its static content (application data) with mtime in the query parameter. This is derived from the modification time of these files. Nix sets these times to 1970-01-01 01:00:01.000000000 +0100 to reduce entropy in the store. Hence, fava will serve static file urls with a broken cache busting parameter.

This feature should be patched to use the store hash instead of mtime for cache busting.

static_url definition

The cli test can be skipped if you can manually confirm that the cli starts.

I confirmed that running fava from the cli works. And I figured out why the test for static_url fails: it looks for files that are not included in the PyPI archive.

You're right it won't work with Nix, though. How about something like this (untested)?

    file_path = os.path.join(app.static_folder, filename)
    nix_store = "@storeDir@" # substituted with builtins.storeDir
    if os.path.commonpath([file_path, nix_store]) == nix_store:
        # Use the store hash for cache busting for files in the Nix store
        # (hopefully all of them).
        values["nixhash"] = os.path.relpath(file_path, nix_store)[:32]