Thank you JoePublic. That appears to have done the trick, but I can't say for sure because there have been short periods of time when that red outline went away as I worked, then came back for no apparent reason. If this is successful it'll save a project that was fast becoming impossible. I'm not a young person and focusing my eyes close-up is not as easy as it once was.

Why do you think they would make an option so basic as using that highlight so difficult to turn off?