docs: note scrollbar differences between Material and Cupertino

[BenjChristiansen]

Description of what this PR is changing or adding, and why:

Adds a short note in the Scrolling section describing visual scrollbar differences
between Material and Cupertino platforms. This clarifies platform adaptation
behavior and addresses the request in #4306.

Issues fixed by this PR (if any):

Fixes #4306

PRs or commits this PR depends on (if any):

None.

Presubmit checklist

• [ X] If you are unwilling, or unable, to sign the CLA, even for a tiny, one-word PR, please file an issue instead of a PR.
• [X ] If this PR is not meant to land until a future stable release, mark it as draft with an explanation.
• [ X] This PR follows the Google Developer Documentation Style Guidelines—for example, it doesn't use i.e. or e.g., and it avoids I and we (first-person pronouns).
• [ X] This PR uses semantic line breaks
  of 80 characters or fewer.

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[gemini-code-assist]

Summary of Changes

Hello @BenjChristiansen, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request enhances the Flutter documentation by clarifying how scrollbars behave differently across Material and Cupertino design systems. The update aims to provide developers with a better understanding of platform-specific UI conventions, ensuring a more native look and feel when adapting applications, and addresses a specific request from the Flutter website issue tracker.

Highlights

• Documentation Update: A new section has been added to the platform-adaptations.md document, specifically detailing the visual differences in scrollbar behavior between Material Design (Android/Web) and Cupertino (iOS) platforms.

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[gemini-code-assist]

Code Review

This pull request adds a new section to the platform adaptations documentation, explaining the differences in scrollbar behavior between Material and Cupertino platforms. The added content is clear and accurate. My main feedback is to adhere to the semantic line break style guide, which is mentioned in the PR checklist, to improve the readability and maintainability of the documentation.

[sfshaza2]

lgtm. Thanks, @BenjChristiansen! That is certainly a finely aged issue!

[sfshaza2]

/gcbrun

[sfshaza2]

Oh, @BenjChristiansen, can you sign the CLA?

[flutter-website-bot]

Visit the preview URL for this PR (updated for commit 5853456):

https://flutter-docs-prod--pr12899-docs-scrollbar-note-nphgu2b3.web.app

[BenjChristiansen]

Thank you for the review and approval! I signed the CLA yesterday 😀

[sfshaza2]

/gcbrun