Can JavaScript detect when the user stops loading the document?

As I suggested in the comments... add the script in the onload event.

Edit: I guess I can explain my reasoning for the suggestion, hopefully it'll help others trying to save the same problem.

A browser will continue to "load" (and thus play the throbber) until it has fired the onload event for every "window" (each DOM tree with a global window parent) in a given window or tab. Many outside resources are able to download in parallel, but by default—and in fact, there is no escaping this default in any browser except IE¹ (using the defer attribute on the <script> tag)—<script> resources will "block" further processing of the document. If the <script> resource request never completes, the page's onload event never fires (and there are other side effects as well, if there is content after the <script> in the DOM: the DOM may never be fully loaded, and resources after that <script> may never load at all), and thus never finishes "loading".

Carrying on from that, you may also be able to improve performance by adding an initial <script> at when the DOM is loaded, with a defer attribute for IE, and cutting the connection for other browsers when you expect the onload event would fire (this is hard to pinpoint exactly and may require experimentation).

¹ To keep this answer up to date... the defer attribute is available in most modern browsers now.

I asked the very same a very similar question a few days ago and the general notion was no.

Looks like the DOMContentLoaded event does the trick in Firefox 3.5 - it's fired the first time the user stops loading the page. After that the Stop button is disabled. The user can still press Escape to stop loading again, but I can detect that using onkeydown.

However, this doesn't work in Chrome - it fires DOMContentLoaded as soon as the page loads, without waiting for my dynamically generated <script> tags.