fix: Fix broken pointer scroll in virtual lists in certain situations

[avinashbot]

Description

See ticket for repro and video. It's pretty easy to reproduce but hard to do by accident (open the thing with the keyboard and scroll with your mouse without moving your cursor). Turns out, it's because the return value of useVirtual isn't stable between renders, so we got accidental rerenders, and scrollToIndex got called unintentionally. Fixed it by only using scrollToIndex (made stable) inside the useEffect.

Related links, issue #, if available: AWSUI-62335

How has this been tested?

Tested in a dev page, verified that it works once I switch to the new implementation. Kiro wrote me some unit tests, but they were complex and mocked basically everything since JSDOM doesn't support a lot of measurement stuff. So integ tests it is (also written by Kiro).

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 97.71%. Comparing base (3b62c18) to head (f2bcfdd).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5124   +/-   ##
=======================================
  Coverage   97.70%   97.71%           
=======================================
  Files         990      991    +1     
  Lines       31818    31838   +20     
  Branches    11752    11758    +6     
=======================================
+ Hits        31089    31109   +20     
+ Misses        722      683   -39     
- Partials        7       46   +39     

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