Anchor scroll offset

Now that the expression for calculating `necessaryOffset` only depends on `getBoundingClientRect()`, we can indeed mock that.
But what's the fun in it ?

I believe at some point it would make sense to have some e2e tests (but putting it in the docs app is kind of too much). Maybe when this #9527 gets implemented, we could add sme e2e tests and make those unit tests more "unit"-y (and robust).
For now I think it's a good thing that we have some minimal interaction with the actual functions.

WDYT ?