[Improvement] Preserve scroll position on diff viewer

[thiagoarmede]

Currently, when we switch threads or even if we switch tabs in the right panel, the diff tab's scroll position is not preserved, which is really annoying when we have large diffs 😭.

I keep switching threads to give answers to the agents and forget that if I come back to the one that I was reviewing before, I won't be in the same place I was before.