fix Web IDE file tree scroll issue

What does this MR do?

Fixes a Firefox scroll issue for Web IDE's file tree.

Why was this MR needed?

#46029

It turns out that the real issue was that the sidebar element's scrollbar wasn't being activated. After some research, this had to do with a Firefox vs. Chrome flex-box implementation.

Screenshots (if relevant)

N/A

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

#46029