Make scrollbar assertions less aggressive

[Piinks]

This adjusts the assertions on interactive scrollbars to only be applied when the scrollbar is visible.

Related issues:

• Fixes Scrollbar throws due to recent assert changes #84735
• Auto-Scrollbars on Desktop are disabled flutter-team-archive/gallery#523

This also adds more documentation and code samples. I cleaned up a bit, removing outstanding todos and fixing some abbreviations in the tests.

For some of the removed todos, I filed issues to actually track the feature requests. 🙃

• Expose more properties on RawScrollbar #84793
• Add support for a RawScrollbar.shape #84792

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• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt.
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[davidmartos96]

I like the proposed changes. Delaying the assertion to when the scrollbar has been interacted with, will help a lot to tell which scrollbar in the tree is incorrect. The error in the gallery project was a bit confusing.

[Piinks]

Delaying the assertion to when the scrollbar has been interacted with, will help a lot to tell which scrollbar in the tree is incorrect.

This should only delay it until it is visible and available for interaction. :)

[dnfield]

Thanks for the example, really helpful

Lgtm

[fluttergithubbot]

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