R1:325682248a8e

If there's an anchor in the URL in Differential, don't stick to the bottom of the page as content loads

Summary:
Fixes T11784. A lot of things are interacting here, but this probably gets slightly better results slightly more often?

Basically:

- When we load content, we try to keep the viewport "stable" on the page, so the page doesn't jump around like crazy.
- If you're near the top or bottom of the page, we try to stick to the top (e.g., reading the summary) or bottom (e.g., writing a comment).