On 20 Mar 2012, at 15:20, David Vereschagin wrote: > I can't figure quite out what triggers it but it does seems to have > something to do with whether a message from a mailing list is > currently selected in the messages pane.
This has been the case for me as well. I've noticed that it doesn't do this for *all* mailing lists, just some of them. The common factor appears to be the presence of a 'List-Post' header in the selected message: if I'm displaying a message that has this header when I compose a new message, the value of that header gets pasted into the To: field. This is definitely new behavior in the latest beta.
