A recallable default-anchor provider — the function form of FocusActivation's initial
anchor. Returns a fresh default anchor (an Element resolved through the region walk, a fixed
AnchorSource, or null), synchronously or as a Promise. Invoked at cold-start and re-invoked
whenever the live anchor's element is destroyed (the destruction fallback — see the class doc).
A recallable default-anchor provider — the function form of FocusActivation's initial anchor. Returns a fresh default anchor (an
Elementresolved through the region walk, a fixed AnchorSource, ornull), synchronously or as aPromise. Invoked at cold-start and re-invoked whenever the live anchor's element is destroyed (the destruction fallback — see the class doc).