Scrollytelling fix

[gjmooney]

Description

This fixes the janky scrolling some people were experiencing using story maps in Specta. Segments were skipped sometimes, or one scroll would cause multiple segment changes.

I think the scroll issues were caused by scroll position being computed from stale layout during segment transitions, especially when repaints were slow. This PR adds a callback from StoryViewerPanel to MainView that runs when segment transition animations finish, and blocks further scroll-driven segment changes until then.

Checklist

• PR has a descriptive title and content.
• PR description contains references to any issues the PR resolves, e.g. Resolves #XXX.
• PR has one of the labels: documentation, bug, enhancement, feature, maintenance
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📚 Documentation preview: https://jupytergis--1128.org.readthedocs.build/en/1128/
💡 JupyterLite preview: https://jupytergis--1128.org.readthedocs.build/en/1128/lite

[github-actions]

👈 Launch a Binder on branch gjmooney/jupytergis/scrollytelling-fix

[github-actions]

Integration tests report: appsharing.space

[martinRenou]

Thanks!