For some reason, the glitch doesn't occur when Blender is left free-floating: if I set it to fill the screen or either half of the screen, the glitch happens again. I can't get into system preferences or anything, either, because any pop-up from Blender has the same problem but irreversibly.
I don't know what changed. The only marginal thing I could think of is I installed a new library to my Anaconda IDE for Python, but I have the zipped Blender: they each have their own bundled version of Python.
I have a Windows 10 64-bit O.S., if that helps. But again, I don't know why this glitch is happening now when Blender was fine just a couple days ago. I have tried uninstalling and reinstalling Blender, but it doesn't help...
Edit: I have tried downloading a fresh copy from Blender's website; I've tried ZIP bundle AND msi executable downloads; I've tried multiple versions of Blender; and I have tried x32 vs x64 variants of each version. The glitch has persisted throughout.