Ctrl+Backspace teleports text cursor

When pressing Ctrl+Backspace to delete the previous word, Element incorrectl teleports the text cursor to the end of the input box; see video:

https://user-images.githubusercontent.com/399535/125371658-806f5b80-e381-11eb-8569-b97426a2a78b.mp4

Browser: Firefox 89 on Linux

#### Steps to reproduce

Minimal repro:

Input: a, Shift+Enter, b. You obtain in your input line the multiline message:

```
a
b
```

Place the text cursor behind `a`, and press Ctrl+Backspace. (On OSX a different keyboard combo might delete an entire word this way.)

Now instead of just deleting the `a` and placing the cursor in its stead, it deletes the `a` and teleports the text cursor away.

#### Version information

For the web app:

- **Browser**: Firefox 89
- **OS**: NixOS Linux
- **URL**: app.element.io