What optimizations does a browser perform when a DOM element is being rendered off screen?

Question

I control an iframe being rendered on another domain. Is there a clear way of telling whether this iframe is being rendered above or below the fold on the parent page? Obviously, I can't access the parent DOM because of the same origin policy restriction.

For example, I tried measuring the speed of rendering on a canvas or VML tag, hoping it would be faster when the tag isn't on screen. This didn't work - there was no discernable speed difference. Ditto for DOM manipulation - there doesn't seem to be a way to differentiate between reflow/repaint operations below or above the fold.

What about flash? I don't have much experience, but could one check how quickly a browser renders flash operations? Or does flash have any 'onvisible' events outside of the DOM standard?

Answer 1

I don't think so.

The same origin policy restriction prevents that, although you can use url parameters, which can be set by the javascript embedding the ad, to the offset on the page.

Answer 2

According to the author of SoundManager2 Scott Schiller, there's a slight difference in the latency of the JS->Flash communication bridge depending on whether the flash movie is on screen or not. That is, it's slower when it is not on screen.

Answer 3

This really varies by-browser, as different browsers interact with the Flash plugin differently. The most precise explanation I could find is on an old connectedpixel post.

At the point of writing, Firefox didn't init a flash object until it was scrolled on page. IE, on the other hand, resized the Stage element when it scrolled into view. No versions of browser, Flash, or OS are given, so this may have changed since that test.

In short, I need to make a flash object and play with it, watching for various events.