Accessibility
TakeoverDialog is a modal dialog. The accessible name is title. The heading is an h2 with that same text. description, when set, is the accessible description. The close control's name is closeLabel.
scrollRegionLabel when it overflows and holds no focusable control, so a keyboard user can reach it.start, the start region scrolls beside the body. Pass startScrollRegionLabel when it overflows and holds no focusable control.headerActions with its visible label or Button label.details.cancel() when a parent owns the close, such as a confirm dialog inside the body.
The page does not scroll while the dialog is open. The body is the scroll region. With start, the start region and the body each scroll on the block axis. The title, the description, and the footer stay in view.
The dialog is in the trigger's owner document, including a pop-out opened with window.open. It renders where it sits in the tree, so it takes the theme of the region around it.