I've answered my own question here. When I had the "OH Hat" Track selected in the Arrange window, but was working on the a region from the "Kick Drum" track in the Editor, it would revert to the "OH Hat" Track because it was the one selected in the Arrange window. Later, when I was working in the Editor, an Undo would revert the Editor to the 16" Tom track. That was when I noticed that the "16" Tom" Track was selected... and then it clicked for me. If I'm working in the Editor on a Region from Track X, then I have to have Track X selected in the Arrange window. If Track Y is selected in the Arrange window, but I'm working on a Region from Track X in the Editor, then an Undo reverts the Editor window to Track Y. Get it? Got it. Thanks for your help. You're Welcome.