zxing has an online decoder -- https://zxing.org/w/decode.jspx -- but I've seen it fail so often that I'm wondering whether it's very "strict" / doesn't account for image distortion that much?

Or does the web site use hardcoded settings that are more tweakable in the library?

Huh, interesting -- zxing is in "Maintenance Mode Only": https://github.com/zxing/zxing

I'd be suuuuper interested to know: what's the state of the art in terms of QR decoding, both in terms of decoding speed, amenability to various image distortions, and (which I've always dreamed of) the ability to partially decode a QR code, even if the error correction fails for some parts of it, or even totally? i.e. a very very verbose debugging/decoding mode?