Old private messages are marked as read without any user interaction

[WesleyAC]

After some amount of time (usually a few weeks or months), private messages are marked as read without any user interaction. This appears in both web and mobile. The messages are subtracted from the unread count, and when I go to the narrow of PMs with that person, the messages are not shown with a blue bar on the left fading away, as I would expect if the messages were being marked as read when I opened the message.

This appears to happen in chronological order (that is, if I have 4 unread PMs from different people, the oldest ones will disappear first).

[adi-001]

Same happened w/me too

[timabbott]

One theory is that this is a side effect of #10886, potentially along with Zulip windows reloading when new messages appear -- we might eat your oldest unread message due to a background reload corner case, and these happen to be PMs.

I attempted to test this theory as follows, without luck:

• I tried using reload.initiate -- the mechanism through which Zulip reloads content after a server upgrade. Our logic correctly preserves one's scroll position and did not mark anything as read.
• I tried Ctrl+R browser reloads while viewing "All messages" view. This also did not mark anything as read, though it did select the first unread message, so I imagine further interaction would have marked the first message as read.
• I tried using the browser's "Reopen closed tabs" feature, same result as the previous one.

Ideas of further possibly low-frequency workflows would be appreciated. But possibly we may accidentally fix this when we resolved #10886.