Result dropdown is static and does not move realtive to viewport

**Do you want to request a _feature_ or report a _bug_?**
Bug

**If the current behavior is a bug, please provide all the steps to reproduce.**
* Browse to https://community.algolia.com/docsearch/
* Minimise the height of the browser
* Search for something. Say "test"
* Notice there are multiple search results
* Try to scroll through the search dropdown

**What is the expected behavior?**
The search dropdown should scroll.

**What is the current behavior?**
The dropdown container is fixed and is not scrollable. Instead the website in the background starts to scroll.

I saw this bug first on https://eslint.org/. I realised that when I search for something with multiple results (try something very generic like "variable"), it flows out of the viewport and the container itself is not scrollable. I figured it is an issue with eslint but upon digging a bit, I realised that the bug is coming from DocSearch. Since I could not query on DocSearch which will overflow my browser viewport, I tried resizing the height of browser and voila, bug reproduced! I tested the same behaviour across multiple websites that use DocSearch (react, bootstrap etc) and I could verify the reproducibility of this bug.

[1]: https://github.com/algolia/docsearch-configs
[2]: https://community.algolia.com/docsearch/config-file.html
[3]: (https://jsfiddle.net/)

Reproduce the issue at the DocSearch demo URL by reducing the browser height and searching for a broad term such as "test". Trace the result dropdown behavior from there; done means the dropdown remains within the viewport, scrolls through multiple results, and the page behind it does not scroll.

Written by the indexing model from the issue text.