Including drop down item causes editing side to autoscroll to bottom

### Is this a hackmd.io issue?

- [x] Yes, the bug happens on https://hackmd.io

### What's the problem
I have a long note with a lengthy references section, and would like to make a hideaway menu for it, so that it doesn't have to be open and taking up lots of length when opening the page. It is on the bottom of the document, at the end. When including a dropdown item in the note, the left hand side (the editing mini-page) goes to this part of the script upon autosave/inactivity. It can be very difficult when you're trying to write, as the page constantly jumps to the bottom for no reason, away from the content you're trying to write.

Here is the dropdown item I use:
```markdown

References

- Here we have some things in a dropdown item that I would like to include (such as references that don't need to be open at the beginning)
- Another list item


```

#### Current behaviour
The editing side of the side-by-side preview and raw text page automatically jumps to the bottom of the raw text after the author has not been writing for a few seconds/after a few seconds of inactivity.

#### Expected behaviour
The editing side of the side-by-side page does not move or automatically scroll for any reason while trying to write.

#### Environment

I use HackMD on:

**Desktop**
- Ubuntu 18.04
- Chromium version 80.0.3987.163

Reproduziere das Problem in HackMD mithilfe des bereitgestellten -Markdowns und einer langen Notiz und warte dann im nebeneinander angeordneten Editor auf Autosave oder Inaktivität. Verfolge das Scrollverhalten des Editors im Zusammenhang mit Autosave; fertig ist es, wenn der Bearbeitungsbereich an der aktuellen Position des Autors bleibt, statt zum Ende zu springen.

Vom Indexierungsmodell aus dem Issue-Text verfasst.