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.
Same-document navigations now commit synchronously in WebContent, while
the UI process mirror learns about them over asynchronous IPC. A stale
UI seed could be accepted back into a live non-initial document and
overwrite its latest entry, making queued traversals target unreachable
entries.
Share descriptor comparison helpers between LibWeb and LibWebView.
Reject stale top-level seeds against the active document latest entry,
and let the UI process adopt WebContent current snapshots when a seed is
rejected. Test-only session history dumps now first send WebContent
current state synchronously, so dumps observe the converged state.
Allow post-load UI seeds to carry UI-owned nested histories that the
freshly loaded top-level document has not reconstructed yet. Add unit
coverage for matching those seeds while still checking top-level state.
Add a browser-side model for top-level history entries and history step
coordinates. This gives the UI process a structure to mirror WebContent
history across process swaps.
Add debug dumping support alongside the model so traversal state can be
inspected while working on back and forward behavior.
