Problem: While a document was concurrently loaded, a burst of sync same-
document history navigations (e.g. a pushState flood) could spin the UI
process at full CPU — and on slow/Sanitizer builds, intermittently time
out other tests (since one WebContent process is reused across tests).
Cause: The UI process keeps an authoritative session-history mirror, and
merges each WebContent snapshot into it. find_merge_anchor compares each
local entry against each incoming one. Every URL comparison serializes
both URLs. When a snapshot briefly diverges from the mirror, the anchor
is no longer near the end. So, the search degraded to a deep quadratic
walk — with a string serialization per-comparison. A flood compounded
that from both ends: Every pushState added a top-level entry — driving
the count each walk must cover into the hundreds — and also triggered a
history update. So, the merge ran again for every one of them.
Fix: Index the incoming entries by serialized URL once — keyed as URL
equality compares (full serialization, fragment included). So, the URL-
keyed anchor searches are linear, not quadratic.
766bc991ed