here is a fiddle: http://jsfiddle.net/honk1/jAJfc/1/ its a large scale background image and a fixed header. header and background have the same background image. header has fixed position to be visible during scrolling, so the header may cover the site content that scrolls below it. when the user scrolls and the content has a flash object in it, and it "hits" the header, the image inside the header gets repositioned / jumps to a weird position. in the nightly build its even worth, i think the background-attachment: fixed; attribute isnt set yet..? i mean: its not working in the first place.
of course, if no flash object is inside the content, the behaviour is as desired. same with mozilla/ie browsers.
this also happens on the ipad, where no flash element is placed, but a video element. the jsfiddle is a static html source code from a rendered site. the code gets generated by the longtail jwplayer and should be fine. on mobile devices the player generates a html5 video element. if the video element hits the header, its background image moves, or whatever happens there..
Seems like the background origin is being incorrectly computed when we render into a composited layer.
I've got a similar problem with background-attachment: fixed and loading assets. Set plugins in Chrome to 'Click to Play'. Visit this fiddle. http://jsfiddle.net/CaseyLeask/3s2GE/ scroll the page, with the background working correctly. Load the Flash Player instance. Scroll the page again.
Afterwards, highlight the text for dramatic effect.
*** This bug has been marked as a duplicate of bug 65793 ***