Target 1
Identity boundary
The source element, child Window scroll context, and body render host belong to one active realm.
Tracked content
This element is the source query scope. The rail is its sibling because the child body is the render host.
Target 2
Lifecycle boundary
The parent unmounts its reusable Tracker before this iframe navigates to a replacement document.
Target 3
Styling boundary
The child bundle imports the public stylesheet because parent CSS cannot style this document's rail.