The problem has to do with hidden exposures whose hypothetical rows can't
immediately be made visible either because they would overflow the filled-up
view or because a misplaced result was seen.
The relevant part of `#updateResults()` is [here](https://searchfox.org/mozilla-central/rev/6a2a2a52d7e544a2fd5678d04991a7e78b694f22/browser/components/urlbar/UrlbarView.sys.mjs#1289-1291). When `canBeVisible` is false,
we effectively forget about that hidden-exposure result, which is not right. For
normal results, we also have that `canBeVisible` logic (a few lines down from
the part I linked), but the difference is that we create a hidden row for them
and then unhide the row when the query finishes and stale rows are removed.
We're missing an analog to that logic for hidden-exposure results, so we're
undercounting them.
Fortunately it's not too hard to fix. I added a new kind of exposure for
`TelemetryEvent` to track called "tentative" exposures. Tentative exposures are
entirely analogous to this case where a row would be added but hidden, and then
unhidden when stale rows are removed.
While I was modifying the test, I rephrased the comments for existing tasks so
hopefully they're easier to understand.
Differential Revision: https://phabricator.services.mozilla.com/D205155
a17188bd2b