These tests have been randomly failing on CI. I'm not entirely certain
why there is a race for these conditions. I'm not sure how
browser-ui-test waits when the document reloads on navigation.
There's also some javascript triggers that happen after the page loads,
and I don't remember if that runs before DOMContentLoaded.
Fixes CI failure in sidebar-scroll.goml where mdbook-sidebar-scrollbox custom element not found (timing issue in headless Linux Chrome). The class is always present in generated HTML (<mdbook-sidebar-scrollbox class="sidebar-scrollbox">).
This fixes an issue where the sidebar was scrolling incorrectly when
clicking on chapters when a heading nav is currently in view. The
problem was that it was storing the scrollTop of the sidebar, but it was
not considering that when navigating to another chapter that the heading
nav of the previous chapter would disappear.
The solution is to keep of an offset instead so that it can make sure
that the vertical position of the clicked chapter stays in the same
relative position when the new page loads.
Fixes https://github.com/rust-lang/mdBook/issues/2967
