Backspace in draft makes view jump around

[mordae]

Steps to reproduce

• I input first paragraph of http://lipsum.com/ generated text into the draft field.
• I move cursor to the first line.
• I delete a single letter of the second word.
• View shows the ending of the draft instead of the cursor area.
• I input a single letter and the view jumps back.

Version information

For the web app:

• Browser: Firefox 59
• OS: Fedora Linux
• URL: both http://riot.im/app/ and http://riot.im/develop/

[lampholder]

This is a dup of #6825

There's a reimpl of the composer (using slate.js instead of draft.js) in flight, so hopefully that will wrap up a lot of the issues like this.