Failed to fillFrame: Frame is missing uuid.

Hi there, thanks for looking into it!

if I autofilled the fields, left the checkout page and returned to my cart, and then clicked "Checkout" again, the fields would be pre-filled with what I had autofilled prior to leaving the checkout page.

Ah this is part of the WooCommerce Blocks functionality, it saves your data in a draft order so when you return you don't have to fill it in again.

Interesting discoveries you've made, what I am seeing is that when going via the browser extension button and autofilling from this panel

the onBlur event is fired.

When using the autofill this way, via the autocomplete box attached to the input it isn't.

You can execute this script on the checkout page in the JavaScript console to listen for onBlur events and see which fields and what the values are when they're blurred.

js
const x = i => console.log( { id: i.target.getAttribute( 'id' ), value: i.target.value } ); Array.from( document.getElementsByTagName('input') ).forEach( a => a.addEventListener( 'blur', x ) );