We learned that the underlying issue was related to react-dom's SyntheticEvent.augmentClass function being undefined.
This seems to be caused by attempted property assignment after the SyntheticEvent had been replaced by a Proxy of itself. This works fine in Chromium et al, but QtWebKit doesn't deal with Proxy Event objects well.
Moving the augmentClass definition and assignment up above the Proxy stuff resolves the issue in a PR to React: https://github.com/facebook/react/pull/10011