Describe the bug
With multiple pure-documentation MDX docs, the scroll position of the preview window does not reset when switching between docs.

To Reproduce
Steps to reproduce the behavior:

1. Create at least two MDX docs without stories, with content long enough to result in a scroll bar.
2. Select the first doc in the sidebar.
3. Scroll down the first document.
4. Select the second doc in the sidebar.

The second doc loads with the scroll position already partially down the page.

Expected behavior
Selecting a doc should always start with the scroll position at the top of the page.

Screenshots

Code snippets
https://github.com/zolk/storybook-mdx-scroll-bug

System:
Reproducible in Storybook 6.0.21.