This already exists within the frontend toolkit, use both stick at top
when scrolling (to set the page contents to position:fixed) and stop
scrolling at footer, to return the page contents to position: static
when the top of the footer reaches the bottom of the page contents.

The navigation highlight module uses this hook for the
in-page navigation (js-page-contents).

Also add a js hook to the page contents area
This is required to get the stick-at-top-when-scrolling js to work.

Stick at top when scrolling also has a futher optional hook to set the
sticky element and the shim to resize to fit the parent (the hook for this is js-sticky-resize).

These are required by GOVUK.stickAtTopWhenScrolling.

Add extra padding on to the bottom of the page contents list, so that
it doesn’t allow the Is there anything wrong with this page? link to
overlap the bottom of the page contents list.

When the page is scrolled, this will highlight an item in the left hand page contents.

- it has no highlighted nav items, when the page loads
- it highlights a nav item, when the page is scrolled

Mock the values for scroll position and heading position, and also
footer position to ensure the last item is highlighted.

The body of a guide is rendered using a shared ‘GovSpeak’ component.

In test mode shared components are not fetched from Static. Instead they are rendered as a dummy tag which contains a JSON dump of the locals - the arguments passed to the component.

This means that the headings we expect to be present on the page (within the body of the guide) are not present. This which was causing JavaScript errors on the page, which were appearing in unrelated tests that used the Javascript driver to execute.

By returning early if we cannot get the offset of a heading we guard against this.

This is so that when a page contents anchor is clicked on, the title
“Page contents” and the title of the section will line up - rather than
the title of the section sitting at the very top of the page.