chore: hide scrollbars in dev pages when motion is disabled on Safari

[jperals]

Description

When updating Safari from 15 to 16 in our visual regression Safari tests, scrollbars appear and they happen to be flaky —with variable opacity, and sometimes not appearing at all, causing unexpected screenshot diffs. Therefore bringing back scrollbars hiding for Safari in our dev pages.

After this change, scrollbars are not visible when using Safari and motion is disabled.

Note: I noticed that for the setting to have effect, motion needs to be disabled when the page is loaded. So in order to switch scrollbars on or off manually, it is necessary not only to toggle the motion disabled option (either via the checkbox or via the URL parameters) but also to reload the page. This seems to be a browser limitation and does not affect our tests.

This reverts commit f9aeff8 "in essence", but with two differences:

• Hides page-level scrollbars as well, not only those inside the body
• Hides scrollbars only in Safari, does not affect Chrome in MacOs (which is what most devs use to manually test)

Other options considered:

• Disabling scrollbars via feature flags. No such option found for Safari / Browserstack
• Adding a delay in test initialization. This would affect the tests for all browsers and for the sake of covering scrollbars in tests, they area already covered in Chrome tests.

How has this been tested?

Ran in my pipeline. Scrollbars are consistently hidden in Safari components tests.

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 96.42%. Comparing base (22d7bca) to head (8175d31).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3303   +/-   ##
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  Coverage   96.42%   96.42%           
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  Files         791      791           
  Lines       22603    22603           
  Branches     7814     7814           
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  Hits        21795    21795           
  Misses        754      754           
  Partials       54       54           

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