Website content is slightly wider than the brower's viewport when in narrow mode, i.e. less than 980px wide. It turns on horizontal scroll for a few pixels, which is a frustrating user experience.
In the dev tools I found the following media queries:
It looks like the search bar is the one to blame, as hiding with with display: none fixes the page width. Specifically, this element:
For some reason this CSS rule is applied to it: