Mark-unread can fail if topic was concurrently moved

[gnprice]

Scenario:

• You long-press a message, because you want to mark the conversation unread starting from there.
• While you have the action sheet open, the topic gets moved — for example, perhaps it gets resolved.
  • This causes the message list to update to the new topic, as seen for example in the app bar.
• Then you hit the "mark as unread" option in the action sheet.

Expected behavior: The message and the subsequent conversation still get marked as unread.

Actual behavior: Nothing happens. (Concretely, the app sends a mark-as-unread request aimed at the old narrow. Typically there will be no matching messages and so nothing will happen.)

I discovered this bug in the course of making other changes to the action-sheet code. #1044 contains a fix.