"This page has been changed, please refresh and try again" while trying to leave comment

### Is this a hackmd.io issue?

- [x] Yes, the bug happens on https://hackmd.io
- [ ] No. If it's a self-hosted CodiMD instance, please fire up the issue in the repo of CodiMD: https://github.com/hackmdio/codimd

### Read our Official tutorial

It's on https://hackmd.io/c/tutorials/

或是中文版 https://hackmd.io/c/tutorials-tw/

### What's the problem

Attempting to leave a comment when the file has been edited results in one or more "This page has been changed, please refresh and try again" errors. The comment is lost if not copied before refreshing.

This seems to happen even if the other editor is working on a different section.

#### Current behaviour

> Screenshot (optional)

Provide steps to reproduce:

1. Attempt to leave a comment while somebody else is editing the file

Happened at https://hackmd.io/@bee4queer/ByJgVmbJfg

#### Expected behaviour

Another editor working on different parts of the document should not affect the ability to leave a comment.

#### Environment

I use HackMD on:

**Desktop**
- OS: MacOS
- Browser: FIrefox
- Browser Version: https://hackmd.io/@bee4queer/ByJgVmbJfg