Thanks, great to see where I went wrong.
I created the rows using a BlankPanel because when I double-clicked on the repeating panel when it was first created in the data grid, the white area intended to guide users on where to drop components was misaligned with the row panel (this happens only in my full app with all the containers with various visibility statuses, not in the stripped-down clone I provided). When I tried to drag components into the white area, I got the red crossbar.
I’ve played about with it and figured out how I can drop components in there now (ignore the white and dark and drop them directly into the row panel), just was thrown by the ‘guide screen’.
Edit: This seems to have been caused by the data grid being quite a long way down in a long app, or columnised next to another component. When I moved the grid to be the first element at the top of the app and made it span the ‘ful’ width of the app (tremporarily), it made the guide screen align correctly with the row panel.