Ah, in that case I was even more confused about the form than I had realised.
Looking again (now that I have something to look at), I notice the 'Sel' checkbox for the first time - since I had previously only ever dealt with 'all but selected' I'd never needed to look this far along the line, and I guess when I read as far as the folder dropdown my mind automatically switched to 'I ain't gonna need this' mode.
I also notice the 'Simple mode' link for the first time, which changes surprisingly little - but if I could set an option to have this mode linked to in the chatterbox instead of the current 'search inbox' and 'search archive' links I might avoid a small proportion of this confusion in the future.
I do know that as someone that rarely receives, occasionally replies to and never archives messages I have benefited not at all from the changes to this system; I suspect that whatever the powerusers have gained has come at a cost to the rest of us.
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