Reviewed and approved by GPT-6 Astra (highest-capability model available in this session).
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Typing two keys 0–5 ms apart reproduced dropped/overwritten characters during the SwiftUI field focus handoff. The native field positions its caret while gaining focus, and the grid buffers printable events arriving before that handoff completes.
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Selection observation now lives in a leaf rather than invalidating the grid hierarchy. Sync responses without changed cells/styles no longer reassign those maps. Resize response decoding uses the existing background actor. These remove identified unnecessary UI work; no end-to-end large-workbook latency claim is made.
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Native row/column handles preview sizes locally, clamp to server bounds, and submit one asynchronous save on release. Escape or removal cancels unsaved previews; failures restore server-confirmed sizes. Existing menu sizing and column double-click autofit remain available. Handles accept the first mouse event in an inactive window.
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Validation: full Swift test run passed (4 native interaction tests, 12 Swift Testing tests; authenticated live test skipped without credentials). Native tests cover typing at 0/1/5/10/25/50 ms intervals, failed-commit draft preservation, selection, both resize axes, preview geometry, no requests during drag/cancellation, exactly one request after release, confirmed sizes, and failure rollback. Mouse drags route through NSWindow; Escape is sent to the handle directly because XCTest's window remains inactive. iOS simulator build passed. Final rerun after conditional dictionary changes passed; git diff --check passed.
amadan