Scrollable text frame feature.
Currently you have the option in the overlays panel, but it currently doesn't work. This is valuable for many design jobs where the client has to cull text to make the content fit into a specified page size. Particularly in PDF format, so if it's possible to make it work for all export formats, not just ePub and Publish online. Clients are still using PDFs so we need to make things work for them.