Normally, after you mark a message as unread, then leaving the selection on the message will not automatically change it to read, until you move the selection. But if you mark more than one email as unread quickly, then leave the selection on a message, that message is marked read again.
Seems WFM for me.
Wayne asked if I still see this, and I do. Clearer STR: In an IMAP folder setup so that messages are automatically set as READ after one second, with the Read column exposed: Works: Click on the Read column for a single message, and leave it selected for 5 seconds. It stays marked as Unread. Does not work: Select an unread message. In less than one second, mark it unread in the Read column, select the next message, mark it also as unread. Wait a few seconds. Expected: selected message stays unread Actual: selected message switches to read after the unread delay. I'll change it to minor.
It sounds Bug 506109, because "Automatically mark messages as read" is set to a delay of one second.
Not a dupe (predates the code at fault in bug 506109), but also no longer present, probably as a result of the refactoring bug 474701.