This appears to affect any form with an Atto editor. When a form field which is not the Atto editor has keyboard focus, then pressing enter should submit the form. Instead it Opens the font menu of the atto editor.

Here are two example steps to reproduce.

1. In a forum, start a new discussion (e.g. https://moodle.org/mod/forum/post.php?reply=1714304#mformforum).
2. Ensure there is some content in both required fields.
3. Get keyboard focus into the Subject field.
4. Press enter.

Expected result: The form is submitted
Acutal result: the 'Font' menu in the atto editor opens.

The same happens in the quiz, when manually grading questions (as explained in the forum thread I linked to above.)

1. Create, or find a Quiz with some student attempts.
2. Go to Results -> Manual grading.
3. If necessary, turn on 'Also show automatically graded questions'.
4. Click to grade some responses.
5. Enter a mark.
6. Press the enter key.

Exected/Actual as before.