As discussed in https://github.com/whatwg/html/issues/7345, an opener that remains on a popup that error'd because of COOP+sandbox can lead to guessing URLs cross-origin using history length. Instead, the spec changes in https://github.com/whatwg/html/pull/7364 to enforce COOP, even when we'll fail afterwards, severing the opener.
<rdar://problem/86152095>