Inbox collapse can trigger 1s hang

[gnprice]

See report in chat: https://chat.zulip.org/#narrow/channel/243-mobile-team/topic/inbox.20view.20performance/near/1984835

I have a channel that gets all my GitHub notifications, and thus has like 5000 topics with unread messages. The inbox view mostly works fine with it, but a thing I've noticed is that when I click the toggle to collapse the channel's 40K unreads, the collapse animates instantly but then immediately after there's like a 1-second lag where the app feels frozen. After that it works happily.

We don't have a diagnosis for this. Initially I thought it might be #1065; but the slow computation in that issue has to be completed in order to even tell the collapse animation to begin, and the lag here comes after the collapse has finished.

I wasn't able to reproduce this. I tried a test user that's subscribed to basically the set of default channels on chat.zulip.org, and where I marked the last year or so of history as unread (well over the 50k limit of what the server will tell the client anyway). It's possible that the key to a repro is to match the reported pattern where one channel has an enormous number of distinct unread conversations.

It's possible this is a symptom of #389; not sure.