Function seen_binding 

pub fn seen_binding(component: &str) -> String

__SEEN_<C> — the key of every instance of C that has ever rendered, mapped to the PAINT PASS it last rendered in.

★ THIS IS THE VDOM SIDE OF THE PAGE, and it is what makes When this component disappears: answerable without asking the host anything. A paint stamps every instance it draws with the current pass number; after the paint, an instance still carrying an older number was not drawn, which means it left the page. The DOM says what is on screen and this map says what the program believes is on screen — the two linked, so a diff between them IS the unmount set.