Open threads at the latest message, and keep the pane visible while switching #17687

Problem

Switching back to a thread can leave the viewport above the newest message (#12372, triaged as a valid bug; #5903 is the same family). The current workaround remounts the list to force bottom-on-entry, which blanks the pane while the new list measures its rows.

Proposal

I have an open pull request, #16930, that does two things:

1. Ordinary thread entry opens at the latest message. Scrolling up inside an open thread keeps the reading position, and search and citation navigation keep their explicit targets.
2. One list viewport stays mounted across thread switches, and the outgoing thread stays visible while the next one loads, so the pane never goes blank.

No new setting or wire field; the change is in six apps/web files. Native mobile is unchanged. I did not run the packaged desktop app.

Before and after

Dark mode, Iris theme, 1280 × 800. Scroll up in thread A, switch to B, return to A.

Before (previous revision of the PR):

After:

DOM sampling on animation frames found 11 empty samples before and none in 214 samples after. That checks these transitions only; it is not a frame-rate or CPU measurement.

What I would like to know

1. Is "ordinary thread entry opens at the latest message" the behavior you want? It differs from restoring a saved reading position when you return to a thread (the approach of the closed fix(web): return to the saved scroll position after switching threads #14212 and the open fix(web): reopen a thread that was following its stream at the end #14916). If you prefer restoring, this PR's entry behavior should change.
2. Is keeping a single viewport mounted across thread switches acceptable, rather than remounting per thread?
3. The PR is large (6 files, about +950/−460), and fix(web): reopen a thread that was following its stream at the end #14916 and fix(web): recover timeline following and bottom navigation #15859 touch the same following and bottom-navigation code. Would you rather I split it, or wait for those to land first?