<!doctype html>
<input type=text>
<script>
  let input = SpecialPowers.wrap(document.querySelector("input"));
  SpecialPowers.Cc["@mozilla.org/satchel/form-fill-controller;1"].getService(SpecialPowers.Ci.nsIFormFillController).markAsAutofillField(input);
  input.getBoundingClientRect(); // previewValue setter depends on the reframe posted by markAsAutofillField() having being processed...
  input.previewValue = "Autofill";
  SpecialPowers.wrap(window).windowUtils.addManuallyManagedState(input, "autofill");
</script>