The code relied on an IntersectionOberver, so the ToC was only updated when a heading was in the viewport.
It meant that if we jumped to a part of the text that has no heading, the ToC would still point to the old entry.
The new code looks for the correct heading when the view is scrolled so the correct entry is always selected.
Signed-off-by: Kévin Commaille <zecakeh@tedomum.fr>
