Here is a link to a video of the problem: https://vimeo.com/960221412/2a2a524fd6?share=copy
I have tried creating new tracks and track folders to see if this helps. I have moved tracks out of folders to see if they display. (They don't.) I have quit and reopened DP. I do not know what else to do.
I know I hit some key or key combo by accident, and caused this. I have searched the forum here, but unfortunately my search terms (track selector problem; sequence editor track display) are too general to yield helpful results. The DP manual offers little help, I've looked in the sequence editor section and under track selector.
If someone recognizes this problem (possibly it's a feature, not a bug, but gah,

Much gratitude to all here who help so many,
-Helen