The cursor doesn't move when using Chinese input method

### 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
The cursor doesn't move when I want to select the correct word in Chinese input method.
當使用中文輸入法時，如果將游標按左鍵往回，游標不會顯示移動。

#### Current behaviour

> Screenshot (optional)

Provide steps to reproduce:

1. Open a document
2. Use Chinese input method to write some word
3. Back to select word
4. See error

#### Expected behaviour

The cursor doesn't move. In the screenshot, although I back 4 characters, it still display on last one.
游標不會移動，從截圖中可以看到，儘管我選擇「游」這個字，他的游標仍然在最後一個字。這對於寫作會有些困擾。

#### Environment

I use HackMD on:

**Desktop**
- OS: Linux Mint 19.10
- Browser: Firefox
- Browser Version: 70.0.1
- Input Method: IBus 1.5.17 Chewing

#### Additional context

Add any other context about the problem here.