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applyTimelineStackingReorder resolves the live clip from the preview IFRAME,
then gated it with `element instanceof HTMLElement` against the MAIN window's
constructor. Cross-realm instanceof is always false, so every timeline z-index
commit silently bailed ("element not live in iframe") — the drag resolved the
right z but never wrote it. Use the element's own-realm HTMLElement constructor
(matching timelineDOM.ts). Verified end-to-end: dragging a card down now lowers
its z-index and reorders the row, leaving sibling z-indexes untouched.
Unit tests missed this because the happy-dom test iframe shares a realm; caught
via a real-browser Puppeteer E2E drag.