Can't move cursor to right if formula is first thing in answer

[Eldemarkki]

Steps to reproduce:

1. Create new answer with "Uusi vastaus"
2. Add new formula with "Lisää kaava"
3. Type e.g. "aaa" into the formula
4. Move cursor to before the formula
5. Press right-arrow a few times
6. Type something
7. The text is written before the formula. I would instead expect it to be written after the formula

This happens at least on Firefox 118 on Linux.

Here's a screen recording to better visualize the bug:

[Esinko]

I will track this bug as a firefox specific bug for now.
The behavior of browsers has also changed, so it might be wise to revisit implementing arrow control from a new point of view.