I did find a fix. It wasn't a Roku issue. My TV screen came with a remote that has a Format button that changes the aspect. I'd inadvertently hit it -- possibly by sitting on it -- which changed the image to a lower quality format suitable for VHS players (I have one, for archival tapes I consult) or for old square-format TV movies. Toggling that brought back the correct format for streaming or hi-def. Even if you haven't touched the remote that came with your TV, look for it and try that. (Format adjustment may also be available on the TV itself.)
