Can't tap and hold on Firefox Android in text field to highlight, cut, paste, select all, share, etc

### 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

Can't tap and hold on my phone to paste text, highlight text, select all, cut, copy, etc.

#### Environment

I use HackMD on:
**Mobile**

- Device: Xiaomi Redmi Note 4X
- OS: Android 6.0
- Browser: Firefox v 68.0

#### Additional context

Add any other context about the problem here.

Example of this working as I write this:



This partially works in Chrome, allowing to paste and select all, but not to highlight, cut and copy.


No repository files, tests, or entry points are named in the report. Start by reproducing the tap-and-hold behavior on HackMD in Firefox 68 on Android, compare it with the described Chrome behavior, and trace the mobile text-field interaction until the affected frontend area is identified. Done means text can be highlighted, cut, copied, pasted, selected, and shared as expected.

Scritto dal modello di indicizzazione a partire dal testo della issue.