Restore the larger of the current and remembered offsets, reading further
pages until the remembered offset is reachable. Scroll events that only pin
the body to the end of shorter content no longer overwrite the remembered
offset, and the body disables scroll anchoring so the offset stays a pixel
distance.
Renderers declare sourceLines; a line navigation in a renderer without it,
such as Markdown, is answered without moving instead of staying pending.
Refs #7241
yudshj
1598d8113b