Fix MR diff file tree being hidden behind review bar

What does this MR do?

This MR prevents a scenario where the MR review bar hides part of the diff file tree view. It also fixes a small issue with the system status bar pushing part of the file tree view off the screen.

Screenshots (strongly suggested)

	before	after
without the MR review bar
with MR review bar

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because _____.
• [-] Documentation (if required)
• [-] Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Related to #288343 (closed)