In the latest nightly builds of Fennec, but presumably all older versions too, fixed position "headers" at popular websites like Twitter.com, Google+, TechCrunch, and many more, perform unacceptably slow. The headers scroll with the page, then when the page stops scrolling they sit in that wrong place for a couple of seconds, and then they snap back to the top of the page. This is the single worst layout performance problem I encounter on my couple hours a day of tablet surfing. It's really quite painful -- and I don't say that lightly considering the various other performance issues we've got to tackle. Steps to reproduce: 1. Visit http://twitter.com 2. Swipe up on the screen to scroll the page down. 3. Swipe down the screen to scroll back up. Results: When you swipe up to scroll down, the header disappears up above the viewport. When the scroll action stops, you wait about 2 seconds for the header to return to it's spot at the top of the screen. When you swipe down scroll up the page, the header scrolls with the page, as if it's a part of the normal flow. Then, when the page stops scrolling, the header just sits there in the middle of the page for about 2 seconds before returning to the top of the page. Expected results: The header should never move. It's fixed to the top of the viewport. Only the page content below the header should scroll in either direction. Additional information: I tested this on fairly capable tablets (the Asus and Galaxy devices) and the performance here is pretty awful. I expect it's even worse on under-powered phones. There are clearly other performance issues in Android layout, (I'm tracking quite a few) but this one is visually really bad and it's apparent on some of the top sites in the world. I'm aware of a number of other high-priority items being tackled in layout, but if this was Desktop Firefox, I'd stop ship on something like this and so I think it deserves to be prioritized pretty high up the list.

Doesn't bug 607417 cover this any better?

looks like it to me