[tiensonqin]

Summary

Reuse a cached sparse focus row only when the native responder still matches and that row remains live. A nil or removed row now retries the existing coordinate lookup on the next ownership refresh; a valid cached row keeps its identity even when geometry changes. No new API, timer, pin source, schema, dependency, or default is introduced.

The existing full-width identifier test now requires an active native field editor and verifies that it is the window's first responder before waiting for a pinned-row callback.

Validation: the actual cache-decision source fragment was executed in an isolated Swift counterfactual without AppKit: the base failed nil/invalid-cache recovery, the change passed both, and valid-row identity was retained. The temporary harness was removed. swift build --build-tests passed; existing pure Swift contracts passed 8/8, OCaml build/tests passed 234/234, and schema tests passed 12/12. Strict Swift formatting still reports seven identical public-base diagnostics outside the changed lines; no new diagnostics.

CI: all 16 checks passed. Apple logs explicitly show the full-width callback test passing and a complete Swift summary of 212 tests in 14 suites; the existing bounded native test also passed separately.

Evidence boundary: no AppKit/window test was rerun locally. The stronger native preconditions were compiled locally and exercised by the passing callback test in CI; that test does not deterministically exercise both cache-recovery branches. The earlier pins=[] assertion reproduces on untouched public base, but its actual responder/geometry cause remains unproven; this PR fixes the independently established cache-recovery hole, not a claimed diagnosis of those logs.

Link to Devin session: https://app.devin.ai/sessions/eccad85c243745debb033bc62ba68c4e
Open in Devin Desktop: https://app.devin.ai/desktop/session/eccad85c243745debb033bc62ba68c4e?variant=devin
Requested by: @tiensonqin

[devin-ai-integration]

I'll fix CI failures and address comments from users with write access. I'll skip comments containing "(aside)".

• Disable automatic comment, CI, and merge conflict monitoring