electron/app-frame-guard.mjs

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// A chat app runs in a srcdoc iframe sandboxed with "allow-scripts allow-forms", so its origin is opaque.
// The sandbox stops it from navigating the window or opening one, but not from navigating itself, which
// would fetch any URL: a subframe whose origin is opaque may only load its own document, about:srcdoc or
// about:blank.
//
// Electron 43.7.9: a frame's self-navigation fires will-frame-navigate with frame.origin === "null" and an
// empty frame.url, and preventDefault() stops it before any request leaves.

const OWN_DOCUMENTS = new Set(["about:srcdoc", "about:blank"]);

/**
 * @param {{ isMainFrame?: boolean, url?: string, frame?: { origin?: string } | null } | null | undefined} details
 * @returns {boolean} false when the navigation must be cancelled
 */
export function appFrameNavigationAllowed(details) {
  if (!details || details.isMainFrame) return true;
  // A link to a fragment of the page itself stays inside its own document.
  const url = typeof details.url === "string" ? details.url.split("#")[0] : "";
  if (OWN_DOCUMENTS.has(url)) return true;
  let origin;
  try {
    origin = details.frame?.origin;
  } catch {
    // Electron throws reading a frame whose render frame is gone ("Render frame was disposed…"), as for a
    // frame that navigates again right after a cancelled navigation. A frame nobody can read is not let through.
    return false;
  }
  return origin !== "null";
}