chore: Test that unmountContent is called when app layout is unmounted

[just-boris]

Description

Make sure this case has a test

Related links, issue #, if available: P165315773

How has this been tested?

This is a test

Review checklist

The following items are to be evaluated by the author(s) and the reviewer(s).

Correctness

• Changes include appropriate documentation updates.
• Changes are backward-compatible if not indicated, see CONTRIBUTING.md.
• Changes do not include unsupported browser features, see CONTRIBUTING.md.
• Changes were manually tested for accessibility, see accessibility guidelines.

Security

• If the code handles URLs: all URLs are validated through the checkSafeUrl function.

Testing

• Changes are covered with new/existing unit tests?
• Changes are covered with new/existing integration tests?

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.33%. Comparing base (035ae6b) to head (7da2bb8).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3040   +/-   ##
=======================================
  Coverage   96.33%   96.33%           
=======================================
  Files         779      779           
  Lines       21900    21900           
  Branches     7503     7493   -10     
=======================================
  Hits        21097    21097           
- Misses        751      796   +45     
+ Partials       52        7   -45     

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