Bad/missing previews being displayed in conversation viewer
With the new conversation loading in !92 (merged), a conversation is displayed using the email instances in the conversation that the ConversationList provides to the ConversationViewer. These may contain a bad preview if the full message has not been downloaded (see bgo 714317 for the reasons), and hence the bad previews may show up in collapsed emails in the viewer.
To fix this, the ConversationList's conversations could have their email instances updated when the full message becomes available. This could happen in ConversationMonitor, ConversationListStore, or maybe ConversationListbox, by connecting to
Account.email_locally_complete, but this would be a bit of a hack that should be removed once we are loading previews correctly (i.e when bgo 777118 is imlemented).