FWIW , I installed Sigil on a different system and all is working fine thus far. In my case, I suspect the original problem was related to a bad drive as I had some additional (drive) symptoms appear later.
For the previous poster and others - it is
possible to move the split-view bar all the way to the bottom of Sigil's window (and out of view) which will result in unresponsive split or code view tiles. Unfortunately, my problem was not so simple.