WebDriver API blocking behavior when browser is busy executing some long running JavaScript

Actually, the use of a blocking vs. non-blocking API is a point of contention for many users of the Selenium library. There are a lot of places where the library does a "best guess" attempt at blocking, even on element clicks, but it's not guaranteed. This tension between blocking and non-blocking has been discussed at length among the developer community and is reflected in one of the FAQs in the project wiki. In the case of IE, the driver does attempt to block on element clicks, and it's a race condition whether it succeeds in blocking or not. In the case of your specific page, the IE driver (and Chrome apparently) are "winning" that race, blocking until the operation is complete. However, the driver could just as easily lose that race in other cases, so it's best to use explicit waits for other page changes before proceeding to the next step in your code.

If it becomes a problem in the future, you might be able to mitigate this by setting a page load timeout to move on to the next statement earlier. One small challenge with this approach is that not all browsers may implement the timeout currently (IE does, I don't know about Chrome).