Comment placement is complicated

### 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

### What's the problem

#### Current behaviour

Provide steps to reproduce:

1. Comment on a text section
2. Modify (add or delete) text upstream
3. Try to find out which text section the comment originally referred to

note link: https://hackmd.io/DMPYpovASaifbVCYBsop7Q?view#

#### Expected behaviour

The comment should stick to the marked word(s) or the context of the marked words, if they are deleted. Otherwise it is very difficult for collaborators to figure out after modifications, to which text section the comment originally belongs.

#### Environment

I use HackMD on:

**Desktop**
- OS: Windows 10
- Browser:firefox
- Browser Version: 128.12.0esr

#### Additional context

Add any other context about the problem here.