The WebEngine already allows to get information about the progress of the underlying webkit via the onError() handler and the getLoadWorker() status updates.
However, there are a number of cases where things go wrong in webkit, yet the status of the loadWorker is SUCCEEDED and no WebErrorEvent is generated. A simple case is where a small html page loads a large image. In this case, URLLoader will call twkDidFinishLoading() which is a native call, which in turn will invoke ImageDecoderJava.cpp::setData This call will invoke WCImageDecoderImpl.addImageData (back in the java layer), but when that throws a throwable (which it does, an OOME in resizeDataArray), it will ignore it by calling CheckAndClearException (in ImageDecoderJava.cpp) and the information about the exception is lost (that is, env->ExceptionDescribe() is called which will print thread + exception, but that's it). Hence, at this moment it seems impossible to detect a failure when large images are loaded. While the status of the loadWorker will move to SUCCEEDED, the image won't be shown. There are a number of ways to improve this, so if it is agreed that this can be an enhancement, I'll file an issue for it. - Johan
