We assuming that you are using the latest version of Lightroom. Please turn off the GPU and the try it.
Go to Preferences
Click on the Performance tab
Uncheck the GPU
Restart the Lightroom.
Let us know if it helps.
So here's the confounding thing - it stopped acting that way out of the blue. I didn't uncheck the option to use the GPU, I didn't do anything different - it just decided to work as expected at some point.
Is this a known issue? I'm concerned that without any understanding as to why this happened it could just as easily show up again.
Lightroom CC/6 require a dedicated graphics card with 1GB VRAM, if in case you don't have then you need to disable it from the Preferences to have your Lightroom working properly.