Wrong cursor position after deleting a whole line in a fenced code block

[movsb]

Test input (all vertical bars below are the cursor position indicator):

```
111
222|
```

Now, press ⌘ Delete to delete the second line, the result will be:

```
111
```
|

What is expected:

```
111
|
```

Sorry to have disturbed you for the weekends. Oh, I forgot, it's for Firefox, again 🙂‍↔️.

[jefago]

Haha eventually you'll have found all the ways in which Firefox differs from Chrome. Fix incoming.