R1:d161a07781e0

Improve Differential behavior when scrolling with anchors

Summary:
Fixes T12779. Currently, we scroll down if the midline of the changeset is above the midline of the viewport.

This rule can cause us to scroll improperly when loading changesets //after// jumping to their anchors, since the changeset we want to look at will likely have a midpoint above the document midline. That is, we follow an anchor to `X.c`, then it loads, then we scroll past it.

Instead, scroll only if the changeset is (almost)…