OK... I found out what's happening.
The while line occurs when I have a HIDDEN TAKE FOLDER that is NOT COLLAPSED.
You can't see the track numbers in the images I posted earlier but the white line appears between tracks 66 and 68 (and track 67 is hidden). When I showed my hidden tracks I saw that track 67 was a take folder, and also not collapsed. When I collapsed the take folder on track 67 --and chose to HIDE the track-- the white line was gone.
I'm not yet sure if it's a feature or an anomaly, but... That's what it was!