fix: Prevent dropdown misplacement in iOS

[jperals]

Description

Issue: AWSUI-59635

How has this been tested?

• Manually, on real iOS device and with Chrome's mobile simulator
• Ran the changes on my pipeline
• Added unit 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 96.44%. Comparing base (372abbd) to head (69c6207).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3202      +/-   ##
==========================================
+ Coverage   96.42%   96.44%   +0.01%     
==========================================
  Files         787      788       +1     
  Lines       22191    22186       -5     
  Branches     7555     7613      +58     
==========================================
- Hits        21398    21397       -1     
+ Misses        741      737       -4     
  Partials       52       52              

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[jperals]

In a couple cases (here and 3 tests further down) I'm testing just that a value is set (truthy), instead of the actual value. This is because the value is a CSS expression using calc(), and I don't think it makes sense to be so specific about the implementation. We already have visual regression tests for the expected user-facing end result, which is what matters.