Only mark Thunderbird messages read if they have been opened

I prefer to keep Thunderbird messages bold (unread), until I have opened them by pressing Enter, or marked them read by pressing M.

The only way I have found to get this behavior is go to Edit > Preferences > Display > Advanced, select "Automatically mark as read", select "After displaying for seconds" and enter a very high number in the seconds field.

Now the messages stay unread even after Previewing.