Fix container going full-width when switching from "Side-by-side" "Changes" MR tab before done loading

What does this MR do?

Fix container going full-width when switching from "Side-by-side"/parallel "Changes" MR tab before done loading

• Remove container side-effect from diff script

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

Why was this MR needed?

With "Side-by-side"/parallel diff set: Clicking the "Changes" tab and then before if finishes loading, clicking another tab such as "Discussion" would result in the discussion content going full width.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• 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 #29193 (closed)