In windowed mode, monitoring cursor on editor doesn't equal to practical editor cursor.

### 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
As title.

#### Current behaviour


steps to reproduce:
1. go to **windowed mode** in Chrome,
2. start to edit, but it's not triggered every time.
3. only happened on Mandarin Chinese chewing, haven't happened on English yet.

**Desktop**
- OS: Windows 10
- Browser: Chrome
- Browser Version: latest updated.