Function isBrowserTopWindow

packages/db/src/duplicate-instance-check.ts:7–17

* Check if we're in a browser top-level window (not a worker, SSR, or iframe). * This helps avoid false positives in environments where multiple instances are legitimate.

()

 * This helps avoid false positives in environments where multiple instances are legitimate.
 */
function isBrowserTopWindow(): boolean {
  const w = (globalThis as any).window
  // Exclude workers and SSR-ish shims
  if (!w || !(`document` in w)) return false
  // Avoid triggering inside iframes (cross-origin iframes can throw)
  try {
    return w === w.top
  } catch {
    return true // If we can't access w.top due to cross-origin, assume we should check
  }
}

// Detect duplicate @tanstack/db instances (dev-only, browser top-window only)
const DB_INSTANCE_MARKER = Symbol.for(`@tanstack/db/instance-marker`)