msglist: On event-queue refresh, re-fetch the position you were reading; control scroll position

[chrisbobbe]

When we learn that our connection to the server ("event queue") has expired, after we get a new one, we re-fetch messages in any open message lists because the old data may be out-of-date.

When possible, after the re-fetch, we want to show the user the same messages, in their same positions onscreen, as before. This issue is for doing that.

Currently, after the re-fetch, we always put the user at the latest messages in the view, regardless of what messages they were looking at before.

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First, the code should choose a message to anchor to; see #1008: "But a simple algorithm would be […]".

The re-fetch should be done at that anchor; this requires

• Open a message list in the middle of history, when requested #82

We should also try to put the fetched "anchor" message in the same position onscreen as it was before the re-fetch.

Related issues

• msglist: Control scroll position on renarrowing for bulk message moves #1009