The existing autofill state is set on the form element using nsIDOMWindowUtils::AddManuallyManagedState, but a separate map #filledStateByElement within FormAutofillHandler.sys.mjs is used to keep track of which elements are filled in. However, this relies on input events that don't fire when form elements are hidden. In addition, when a page modifies the form field value directly, the preview state can be modified but the #filledStateByElement map is not.
Instead, remove the extra map, and just use the form field's autofill state directly. The form field will take responsibility for removing the preview state when its value changes.
This behaviour applies to single field input elements and select elements.
Differential Revision: https://phabricator.services.mozilla.com/D211000
