Won't load workflow from PNG, only JSON

I just had a working Windows manual (not portable) Comfy install suddenly break:
- Won't load a workflow from PNG, either through the load menu or drag and drop. Nothing happens at all when I do this, not even an error in the shell or browser console
- Will load a workflow from JSON via the load menu, but not drag and drop. No errors in the shell on drag and drop, nothing on the page updates at all
- Tried multiple PNG and JSON files, including multiple known-good ones
- Pulled latest from github
- I removed all custom nodes. Still have the problem.
- Tried another browser (both FF and Chrome.)
- Tried a private window
- Tried a fresh Comfy install, albeit still running in my A1111 venv, still no joy

I was able to confirm that a fresh portable install does not have this issue, even with the config.yaml pointed to that same A1111 directory for models, etc.