When reading a message, the 'Subject:' header is displayed in a control that doesn't allow any interaction. You can't select portions of the subject, nor the whole of it and copy it to clipboard. The only workaround is to open message source window (View->Message source), locate the 'Subject:' header, select a portion of it that you're interested in and copy. We can't expect ordinary users to be able to find that workaround, nor can we expect anyone to find it comfortable.
Mark it verified/dup.