I’ve tried working around this by disabling add-ons but I still can reproduce. Unfortunately it’s very inconsistent (though it happens to me multiple times in a day).
It seems that when the editor has a field that goes in and out of focus (maybe related to editor_did_unfocus_field in editor.py?), it can trigger the processing progress dialog. Unfortunately I can’t exit out of it or do anything but kill Anki. I’ve seen it happen with the Japanese Add-on as well as the AJT Furigana add-on. I’m on windows 10 64-bit.
It seems similar to this but I’ve seen it happen with more than just the image field for certain: "Processing" when adding images
I can reproduce it in 2.1.45 & 47 as well. Haven’t tried anything beyond that. I did not have an issue on Anki 2.1.35.