Problem/Motivation
When a dialog is opened with the "modal" option, all other elements on the page are hypothetically disabled. Even though those other elements are masked, you can still scroll-through the modal to the body, which is awkward and not expected behavior. This can lead to the user being confused after closing a modal as the page has scrolled past where they initiated the modal originally.
This patch disables body-level scrolling when a dialog is opened as a modal, which should address this problem.
