Function resolveMediaStartSeconds

packages/core/src/runtime/init.ts:497–511

(element: Element, fallback = 0)

  };

  const resolveMediaStartSeconds = (element: Element, fallback = 0): number => {
    if (!element.hasAttribute("data-hf-auto-start") && element.hasAttribute("data-start")) {
      // `data-start` is authored relative to the media element's OWN sub-
      // composition, not the root timeline — `fallback` carries the host
      // composition's resolved absolute start (see syncMediaForCurrentState's
      // inheritedStart), so it must be added, not discarded. Skipping it made
      // a nested video play from root t=0 instead of holding until its
      // parent scene began (issue #1838) — resolveStartForElement's own
      // absolute-expression branch already adds this same host offset, this
      // fast literal-value path just didn't.
      const own = Math.max(0, Number(element.getAttribute("data-start") ?? 0) || 0);
      return own + fallback;
    }
    return resolveStartForElement(element, fallback);
  };

  const isTimedElementVisibleAt = (rawNode: HTMLElement, currentTime: number): boolean => {
    const tag = rawNode.tagName.toLowerCase();