With the way the sticky_header library currently works, if message A is scrolled nearly off the top of the screen and the next item in the message list is message B, then message A's recipient header will be the sticky header. If just a few pixels of message A are within the viewport, then they'll be completely obscured by the header, which means message A is effectively completely off screen as far as the user is concerned, and they're seeing message A's header above message B.

So far, that's all fine — if message B is the next message-list item after message A, then they must have exactly the same recipient header anyway.

But if what comes after message A is a date separator D, then D has a later date than A does. If A is very nearly off the top of the viewport, then A's header will extend far enough to obscure the date in D, and the user sees only A's date. The date separator D must be followed by some message B; the user will effectively see B labeled with A's date.

I believe Zulip web had an equivalent bug for years; we tolerated it then, so it's not a high priority. But it'd be good to fix.

Here's a video @chrisbobbe made to demonstrate this issue (from #480 (comment)):