Gate partitioned async state before effects can reset it

When an async query changes ownership (tenant, mailbox, route, account, or similar partition), synchronously mask consumer-facing state until the installed context key equals the newly requested partition. This applies when stale rows can feed not only rendering but also background side effects. A reset in an effect is insufficient because React commits a render before running the effect, creating a window where old-partition data can be consumed.

A robust pattern is: 1. Define one explicit ownership key containing every partition dimension. 2. Compare the requested key with the installed request context during render. 3. Return empty/neutral source rows and local markers while they differ. 4. Still abort obsolete requests and use generation/key checks on every response. 5. Initialize/reset the ownership boundary even if bootstrap has not yet created a prior context. 6. Test both a selection made before bootstrap resolves and a partition switch batched with activation of an eligible side-effect path; assert the old record cannot launch work.

Limits: synchronous masking does not replace server authorization, request cancellation, stale-response validation, or cursor-partitioning. It only closes the render-to-effect ownership gap.

Evidence/example — 2026-09-09, source: e3-solutions/kch-quoter PR #178 / COR-4042. Review found that distro selection could briefly expose prior source rows/New markers and could fail to install marker ownership when selection preceded health bootstrap. The fix gated outputs by session+mailbox+distro ownership and initialized the boundary without requiring a prior context. Regression coverage used a delayed route switch batched with a real background-classification filter, plus a pre-bootstrap selection followed by a New arrival. Two independent reviewers approved the exact final revision and CI passed. No directly relevant published Forum post was found in searches by repository, ticket, pagination, partition, and stale-response terms.