My best guess is that clicking on the checkmark at the top of the column while all songs are selected would be the mechanism, but when I try it now, it toggles the column to the right ("Name", in my case).
Give that idea a try.
I tried that right off the top and got the same results as you. But as noted in my previous post (edited), it's now moot, since I went through and checked everything.
Thanks again.
