The [persist] attribute requires the element has an id, so it was failing to remember
screenX and screenY for the dialog before this change.
Depends on D63233
Differential Revision: https://phabricator.services.mozilla.com/D63236
This patch also includes necessary JS changes to support this. Most commonly, the dialog was accessed with document.documentElement, which needed to be changed now that the dialog is not the top level element.
Differential Revision: https://phabricator.services.mozilla.com/D52411