I only needed to add a 1px to get it to work apparently! Posting here if someone has the same question :)

onLoadStop: (controller, url) async {
    await injectJavascript(controller);
    await Future.delayed(Duration(milliseconds: 300));

    await _webViewController?.evaluateJavascript(
     source: '''
     document.documentElement.style.height = document.documentElement.clientHeight + 1 + 'px';
      ''',
  );