I developed the first version of Google's equivalent of this (albeit theirs actually computes a constantly rotating key from the environment, it doesn't just hard-code it in the program!).
The reason it has to block until it's loaded is that otherwise the signal being missing doesn't imply automation. The user might have just typed before it loaded. If you know a legit user will always deliver the data, you can use the absence of it to infer something about what's happening on the client. You can obviously track metrics like "key event occurred before bot detection script did" without using it as an automation signal, just for monitoring.
That doesn't make sense. The server would wait to process anything until after you received the signal. If it doesn't arrive within a reasonable period of time that tells you something, the same as right now.
If you mean that you can infer client side tampering with the page contents you could still do that - permit typing but don't permit the submit action on the client. The user presses enter but nothing happens until the check is complete. There you go, now you can tell if the page was tampered with (not that it makes much difference tbh).
The typing actions have to be observed by JavaScript. It's not different to any other JS blocking page load because it's needed for the site to work, that's just how the web works.
This doesn't seem to be the same thing. The article isn't about being unable to type before JavaScript starts executing. If I understand correctly, you're unable to type until a network request to Cloudflare returns. The question is: why not allow typing during that network request? JavaScript is running and it's observing the keystrokes. Everyone understands that you can't use a React application until JavaScript is running. They're asking why the network request doesn't happen in the background with the user optimistically allowed to type while waiting for it to return.
(Separately, I don't think the article has adequately demonstrated this claim. They just make the claim in the title. The actual article only shows that some network request is made, and that the request happens after the React app is loaded, but not that they prevent input until it returns. Maybe it's obvious from using it, but they didn't demonstrate it.)
I don't think that's true in this case; the React application loads first, fully initializes, and then sends its state via Cloudflare request. It can't happen at the same time, by design. It has to happen serially. The article's claim is that you can't type during this second request. Frankly, I wonder if this is actually true at all. The article did not demonstrate this, and there's no problem if you can actually interact as soon as the React application is running. ChatGPT running abuse prevention and React applications requiring JavaScript to work are both uncontroversial, I think.
OK, I haven't looked at the exact sequencing here. But generally, once the action goes back to the anti-abuse service for checking the user can't be allowed to change what they're submitting. The view the anti-abuse system saw has to match what the app server sees.
This perfectly explains the trade-off. But from a pure UX perspective, freezing the input pipeline feels uniquely hostile. They could buffer the keystrokes invisibly in the background instead of locking the cursor, which creates the jarring perception that the site is actively fighting the user.
The reason it has to block until it's loaded is that otherwise the signal being missing doesn't imply automation. The user might have just typed before it loaded. If you know a legit user will always deliver the data, you can use the absence of it to infer something about what's happening on the client. You can obviously track metrics like "key event occurred before bot detection script did" without using it as an automation signal, just for monitoring.