System visibility change reaction code incorrectly tries getting inclusive descendant navigables of a traversable navigable

[awesomekling]

Ran into a type error in the code here: https://html.spec.whatwg.org/#page-visibility

When a user-agent determines that the system visibility state for traversable navigable traversable has changed to newState, it must run the following steps:
Let navigables be the inclusive descendant navigables of traversable.

inclusive descendant navigables takes a Document, but traversable here is a navigable.