R1:e0dbc5752195

Stabilize scroll position as diffs load

Summary:
Try to lock the screen to whatever the user is looking at as we load changesets.

Notably, this improves the use case of taking a known action on a diff. Currently, you have to wait for everything to load or the comments keep getting scrolled down. After this change, the comments stay in the same place on screen.

Test Plan:
Raised the autoload changeset limit from 100 to 1000, looked at a 220 changeset diff.

- Reloaded it while scrolled at the top…