Instant Search: faster mobile search refinements

There are a number of cases where I think we should auto scroll the page for the user to make it faster/easier to see the top search result and refine the search query:
- on mobile, after scrolling down the page (e.g. after a load more). Once the user starts scrolling up a little bit, let's show the search box and the filtering selections so that the user doesn't have to scroll up all the way. A lot of apps seem to do this, and I feel myself missing it.
- on mobile, after a filter is selected further down the list we should rescroll the user back to the top of the page so that we show the first search result. It feels odd to just have the dropdown disappear
- on desktop, the same thing... I think we should scroll to the top of the page if a filter that was further down the page was applied.

The issue names no files, tests, or entry points. Start by locating the instant-search UI and the mobile and desktop filter handlers, then inspect how scrolling behaves after loading more results or applying a filter. Done means the search controls reappear during upward mobile scrolling and filtering returns the view to the top result on mobile and desktop.

Written by the indexing model from the issue text.