I encountered a similar issue awhile back with a Chrome-based terminal emulator, which this would also solve. Every time it outputs a line, it scrolls to the bottom, which forces a reflow; and if the scrollback is long, the reflows get slow, and performance ended up being really pathetic. The solution ended up being a hack to prevent the per-line reflows (Javascript scrolling and bottom-relative positioning), but this would be a better fix.
I've been working on a problem that this browser extension would solve really well. I've got a photo gallery with a hundred thousand photos in it where I implemented near-infinite scroll by creating a really tall div and then drawing about 2x as many thumbnails as are needed to fit in the current viewport, which is updated when the user scrolls. They are square thumbnails so it is easy to calculate exactly where to position each of them.<p>The advantage of this over infinite scroll is that the scrollbar is always there and always accurate, so coming back to the page after following a link works properly. Also, I don't have to send the information about all 100k photos on page load (I can load it with AJAX).<p>A demo of how it works is at <a href="http://beta.hypercheese.com/search" rel="nofollow">http://beta.hypercheese.com/search</a>, and the code is at <a href="http://github.com/jewel/hypercheese/" rel="nofollow">http://github.com/jewel/hypercheese/</a>.<p>It sounds like having this in the browser will allow for much more complex layouts.
I can't tell if this would help rendering large lists of dom elements that are loaded all at once... eg what things like infinity.js, ember tables, etc do. I wish there was a UITableView for the web that was more native than any of those solutions.
The web (and WebKit in particular) is on track to catch up w/ native frameworks in terms of tools at the disposal of developers. Albeit in a slightly more rough form.<p>The next 5-10 years are going to be interesting.