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The reveal used a lagging nullable `revealed` state with a `revealed ?? kind.text` fallback in the caller. Under React 18 concurrent rendering a committed render could observe `revealed === null` while `text` was the full value, so the fallback painted the entire string for one frame before the type-on — an intermittent flash, reproducible on a large Run-all (verified in-browser: 60+ cells flashing). Derive the revealed slice from `text` + elapsed time during render instead of holding it in state. For a non-null value the result is never `null` and never the full string on the frame `text` changes (elapsed ≈ 0 → 0 chars), so the fallback can't fire. `prevText` is tracked in state (not a ref) so a discarded render rolls it back and the change is re-detected on the committed render. Verified via DOM MutationObserver: 0 flashes across 213 animated cells.