tab-content.js

// Please refer to https://dxr.mozilla.org/mozilla-central/source/browser/base/content/tab-content.js#375-393
onPaintWhenWaitedFor: function(forceNonArticle, event) {
  if (!event.clientRects.length) {
    return;
  }

  this.cancelPotentialPendingReadabilityCheck();

  var t0 = content.performance.now();
  var isProbablyReaderable = ReaderMode.isProbablyReaderable(content.document);
  var t1 = content.performance.now();
  dump("Call to isProbablyReaderable took " + (t1 - t0) + " milliseconds. \n");

  if (isProbablyReaderable) {
    sendAsyncMessage("Reader:UpdateReaderButton", { isArticle: true });
  } else if (forceNonArticle) {
    sendAsyncMessage("Reader:UpdateReaderButton", { isArticle: false });
  }
}