Thanks for the help re: drawer removal. Here's a photo of the tab that stops the track. Tab is in left center of photo, slightly high, at the left end of the dark streak where the track rides in the bracket (the parallel, horizontal ridges.) The round button is not the tab. Tab is above it and further left.
I was able to see the tab (one on each side of drawer) easily with a flashlight and bend it outward slightly with a screwdriver. The drawer slid out easily. With the drawer removed, I pried open both tabs a little way - at most 1/16 in. or so. The drawer went back easily and stopped normally after.