iframe.onload doesn't fire for ActiveX controls

Last week I had a few very frustrating days with the web interface of a new application we're building. The web application is basically allowing you to navigate websites as they were at a given moment in time.

The web application is built up from some panels with additional information and a main iframe hosting the actual web page. You type the URL of the page you want to see and presto, there you have it. On the side we'll show you all the other versions of that page we have available. Clicking on one of those versions will bring it up in the iframe.

So far, so good. The problems started when allowing the user to click links in the page that is showing in the iframe. We use a mix of rewriting the HTML before it's shown and intercepting the requests on the server, so that following links actually takes you to the destination as it was at the time that you were looking at. It might sound a bit complex, but it feels a but like time traveling. Imagine the wayback machine, with links working historically.

To update the information in the local panels we hook the onload event of the iframe. And that's where the problems started. Because on some of our systems, the onload event doesn't seem to fire when we open a PDF. It seems to only happen on IE, but not consistently on all our IE systems. A workaround with periodically inspecting the iframe also doesn't work, because somehow we can't even get the URL for the PDFs. And before you ask: yes, the PDFs are served from the same domain as the rest of the page.

It's really frustrating, because when the onload doesn't fire it breaks our UI logic. And the workarounds we've had to do are not pretty and slow the UI responsiveness way more than we'd like.

I've done some searching to see whether other people also had this problem. But of all the questions being posted on iframes, this doesn't seem to be a common occurence. That's why I post it here. Has anyone had the same problem? And if so, what was your solution?