SDKError.fatal

true means the SDK itself is unusable: the engine failed to load (ENGINE_LOAD_FAILED) or the SDK stopped on an unrecoverable fault (ENGINE_FAILED), or init() was never called. Re-initialize, or stop. Everything else is false — the one operation failed and the SDK and the viewer are as they were: a page that did not render, a file that would not open (CORS_ERROR, NETWORK_ERROR, FORMAT_ERROR, PASSWORD_ERROR, SECURITY_ERROR — offer another file), an argument a call could not act on. fatal never says "the document is unusable"; the code does.

viewer.on('error', ({ code, message, fatal }) => {
  if (fatal) showReloadPrompt();
});