While going next/prev over chunkier HTML messages, it got confusing because the body content was not cleared while waiting for those messages to load, so for a moment the new header is shown with the old body. This is exacerbated when a network fetch is done. To address this, I did the following changes, also shows the progress bar while waiting for the body:

https://github.com/jrburke/gaia/commit/54b734d85c2adde7c7951f01d98f96158cd5b63b

would be good to consume that change for the final squashed commit before merging.