Fixes T7033. When we've reframed the main page content we need to scroll relative to the containing frame, not relative to the window.

In Safari, Chrome and Firefox, used j/k/J/K keys to navigate diff content.

Tried some other scroll-based beahviors, like jump-to-anchors.

(It looks like the highlighting reticle got slightly derped a while ago, but it's still functional, so I didn't mess with it.)