This has been making me crazy. I haven't found an Apple OS workaround (other than turning off preview pane), but i found this plugin (open source): http://www.christianserving.org/project/truepreview . It provides switches for customizing the 'read' behavior. Seems to work nicely.
Just go to (menu), (message), (mark), (as unread)
Or, if you do this frequently, customize toolbar, & drag the unread button to the toolbar, & click it at need...
UPDATE. Alas, i had to remove this 3d-party plugin because it was causing some errors with mailbox functioning. So, the problem is still unresolved, in my opinion!
Really? I had installed it and was happy that it seemed to work just fine and solved the issue. I haven't noticed any resulting errors -- can I ask what kind of problems it caused so that I can look out for them?
I was aware that you could re-mark messages as unread, but that's kind of a hassle in my opinion and requires redoing it every time you might click on a message. But thanks for the response.
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