fix inline diff copying in firefox

What does this MR do?

Make copying text from diffs work in firefox. It previously only copied first line

multiselect still isn't possible in diffs though :(

Are there points in the code the reviewer needs to double check?

Screenshots (if relevant)

Firefox:

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [ ] Documentation created/updated
• [ ] API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #31193 (closed)