How can I display a message or loading animation while flask.send_from_directory is processing?

I have a simple web application that calls the Spotify API, and, among other things, allows a user to download their "Liked Songs". The 'download' view writes a csv file of the 'Liked Songs' to a tmp directory and then downloads that file using flask.send_from_directory.

@bp.route('/download', methods=['GET'])
@login_required
def download():
    ...
# Edited with more details below

    return send_from_directory(
        temp_dir,
        target[0],
        as_attachment=True,
        attachment_filename=file_name
    )

It all works as intended but can take a while if a user has many thousands of Liked Songs. During that 20+ seconds I would like to give the user some feedback that the task is processing -- a message or loading animation. I have written a loading animation used elsewhere in my site, but in those cases the view ends with a redirect. I also can reveal a modal with javascript. However, a modal or animation will not automatically close when the "Open Liked Songs.csv" window pops up asking the user to "Open" or "Save", nor after one of those is selected (i.e., when the download view is completed).

Is there a way to trigger an event in the Browser Object Model using the window object for the "Open Liked Songs.csv" pop-up window? I do not know how to identify that specific window. Any other suggestions?

UPDATE: Thanks to @v25 I realize more details are needed. Here is my code again:

@bp.route('/download', methods=['GET'])
@login_required
def download(backup=False):
    ...

    if backup:  # For download of 'Liked Songs'
        playlist_id = '000library'
        file_name = 'Liked Songs.csv'
        url = get_endpoint("get-user-saved-tracks")
        query_limit = 50

    count = 0
    tracks = []

# The Spotify endpoint is limited at maximum of 50
# so I need to iterate with offsets.
# I think this is what is causing the delay.

    while not len(tracks) % query_limit:
        params["limit"] = query_limit
        params["offset"] = query_limit * count
        section = sh.get_playlist_tracks(header, url, **params)
        tracks += section["items"]
        count += 1

    if not tracks:
        flash("Something went wrong. We couldn't get your playlist.")
        return redirect(url_for('routes.profile'))

    with NamedTemporaryFile(
                        mode='w+t',
                        encoding='utf8',
                        newline='',
                        prefix=playlist_id + '.',
                        dir=temp_dir,
                        delete=False,
                                        ) as csvfile:
           ...  

          # Writes csv file to temp directory

    ...
    return send_from_directory(
        temp_dir,
        target[0],
        as_attachment=True,
        attachment_filename=file_name
    )

When I monitor this process with my browser's (FireFox) dev tools 'Network' tab, I can see that it takes a while before status code 200 is returned for my '/download' request. At that point the 'Open Liked Songs.csv' window appears. I think I need some way to trigger my javascript closeModal() when status 200 is returned (polling?). As @v25 suggests below, I am pretty sure it is the iteration of 50-track chunks at the Spotify API that is slowing this down.

Via Active questions tagged javascript - Stack Overflow https://ift.tt/2FdjaAW