P: GPU rendering will not complete in Develop Mode at Fit/Fill zoom levels

Using LR Classic CC 7.3, I've encountered a new problem.  When scrolling through photos in the Develop Module, GPU Rendering will not work at either Fit or Fill zoom levels.  The progress circle in the bottom right spins indefinitely.  When I zoom to 1:1, 2:1, etc., the photo renders almost instantly, which is what I'm used to.  

This happens for every photo, every file type.  Zooming to 1:1 always fixes it.