When we changed client-side routing to be path based, we broke the
behavior that allowed anchor-only links to scroll to the element with
the corresponding id. It would instead cause the webclient to attempt to
load the state and end up on the home menu.

This is caused by the fact that clicking on anchor links fires a
popstate event, but that popstate event has no associated state.

To fix this, we need to be able to differenciate a popstate event from
an anchor click from other popstate events. Initially we considered
simply setting a local variable when the user clicks on an anchor link,
and checking that variable in the popstate handler, but this is both
prone to race conditions and also causes issues when a link is clicked
but preventDefault is called on the click event, preventing the popstate
from occuring.

As such, we decided instead to use the fact that the state on the
popstate event from anchor clicks is null. The state is only null in two
cases: when you click on such a link, or when you use the back button to
go back to the first entry in history where the webclient was loaded. In
the second case, this was purely incidental: we actually parsed the
state from the URL and when the action mounted, it wouldn't touch the
state because the new url was the same as the old one. In order to
differenciate from the anchor link click, we now replace the state when
the urls are identical. This will incidentally save us from parsing the
url again if the user navigates back to this history entry, as the
corresponding state will have been saved.

Enterprise: https://github.com/odoo/enterprise/pull/60741