Weibo is called a single page application (SPA), and the front and back ends are separated and the data is transferred through the API.
I just want to imitate the comment effect, that is, use your own answer, submit the backend api with ajax, and call back to modify the page after saving. The wtform is basically just a decoration, it doesn't matter.

It’s best to post your code together.

Here are my suggestions

When you leave a comment, you must be submitting the form, but you need to let the website know in the redirection after the submission form that what you just made was a comment
So, in the redirection after you submit, you You can add a page=-1 to it
Please see the example below

@main.route('/post/<int:id>')
def post(id):
    post = Post.query.get_or_404(id)
    form = CommentForm()
    if form.validate_on_submit():
        comment = Comment(body = form.body.data, post = post, author = current_user._get_current_object())
        db.session.add(comment)
        flash('Your comment has been published.')
        return redirect(url_for('.post',id = post.id, page = -1))
    page = request.args.get('page',1,type=int)
    if page == -1:
        page = (post.comments.count()-1)//current_app.config['FLASKY_COMMENTS_PER_PAGE']+1
        pagination = post.comments.order_by(Comment.timestamp.asc()).paginate(
            page,per_page = current_app.config['FLASKY_COMMENTS_PER_PAGE'],
            error_out = False)
        comments = pagination.items
    return render_template('post.html',posts=[post],form = form,comments=comments,pagination = pagination)    

The page in the code is used for paging display. You can understand it as the parameter in the url that points to the page.
After he submits the form, what he does is redirect return redirect(url_for('.post',id = post.id, page = -1))
And, url_for can bring **kwargs, so that your request will contain a page=-1
When page == -1, the following statement will pass Calculate (the total number of existing comments) divided by (the number of comments displayed on each page) to calculate the number of pages on the last page

And reassign it to page. At this time, on the comment page below (if displayed in pagination), your comment will be displayed directly to the last page.
I don’t know if you understand what I said.

Use Websocket. The scale of the website is always suitable for websocket. It is not necessary for single-page applications.