fix(theme): nav will not be displayed when scrolling to the top(#1588)

[gaoachao]

Summary

Related Issue

https://github.com/web-infra-dev/rspress/issues/1588

Checklist

• [ ] Tests updated (or not required).
• [ ] Documentation updated (or not required).

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[SoonIter]

LGTM, thanks ❤️️