I've just upgraded to 18.04, and the new version of thuderbird seems to no longer have the option to leave messages marked as unread once previewed. (Unless I'm missing something).
The default behavior is to mark a message as read as soon as it is clicked on. Previously (in the version that shipped with 16.04), I had the option to make it such that a message was marked as read only once replied-to or manually marked.
Is this feature gone, or is it just buried somewhere?