The tricky part is inserting the previous content. It requires a combination of setting overflow:hidden on the container to disable the scrollbar, inserting the previous content, adjusting the top positions, then re-enabling overflow to restore the scrollbars. It has to be done very quickly to avoid visible content jumps and getting it to work well across browser platforms wasn't easy.
Worth noting, scrolling on that site is completely broken on an iPad. Click an article in landscape and it shows an image that covers all but the bottom 50 px of the screen. It's position:fixed, so the actual article text scrolls under it, giving you that tiny slice in which to view four lines of text at a time.
Switch to portrait and at least the text scrolls past on top of the image. But text and image are the same colour, so you can't actually read it.
The tricky part is inserting the previous content. It requires a combination of setting overflow:hidden on the container to disable the scrollbar, inserting the previous content, adjusting the top positions, then re-enabling overflow to restore the scrollbars. It has to be done very quickly to avoid visible content jumps and getting it to work well across browser platforms wasn't easy.