Chinese Input from iPad doesn’t work well.

### 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

### Read our Official tutorial

It's on https://hackmd.io/c/tutorials/

或是中文版 https://hackmd.io/c/tutorials-tw/

### What's the problem
* English input works fine, yet Chinese(or Japanese) input doesn’t.
* Keyboard can’t be shown properly. Sometimes the alternatives of the input goes up to the top of the screen.
* What has been input doesn’t show correctly.

#### Environment

I use HackMD on:

**Mobile**

- Device: iPad (Model A1701)
- OS: iOS 12.1
- Browser: Chrome
- Version: 70.3538.75