I have a use case where I want to run an app on a server and show files/folders on the server in a given directory.

However, the user should not be able to navigate to parent directories above the starting directory.

I've currently implemented it as:

@solara.component
def Page():
    my_dir = Path.cwd()

    def browser_filter(path: Path) -> bool:
        if path.is_dir():
            return path not in my_dir.parents
        return True

    MyFileBrowser(directory=my_dir, directory_first=True, filter=browser_filter)

Which then requires an additional check on new_dir in FileBrowser

            new_dir = current_dir.value.parent
            if not filter(new_dir):
                return

I can add this to this PR or make another one if you think it would be more generally useful.

beautifull fix!
Regarding your last comment, does this solve your problem:
#149
(why is there no pycafe example? :))

here is the link to pycafe

the fix in the issue you mention does indeed work.

However, given that the function of filter is to filter out files and/or folders that should not be shown, wouldn't it make sense to pass the new parent folder through filter first before navigating there?
ie you could consider it to be a bug that filter is not respected when navigating up

I don't think I fully understand the problem, can you show an example that does not work?

import solara
from pathlib import Path

BASE_PATH = Path.cwd()

@solara.component
def Page():
    directory = solara.use_reactive(BASE_PATH)
    def protect_filter(path: Path) -> bool:
        if path.is_dir():
            return path not in BASE_PATH.parents
        return True

    solara.FileBrowser(directory, filter=protect_filter)

pycafe

In this example, based on how you interpret the docstring of filter, you might expect this filter to make it impossible to navigate up above BASE_PATH. But you can access it if you click '..', you just cannot go back. This would be 'fixed' when adding the early return when filter returns False (if not filter(new_dir):)