onPresent property

Called on tap with the anchor of the capsule this item sits in.

Read at tap time, so a bar that moves between the pinned chrome and its route hands out the anchor that is actually on screen.

Null when no capsule can be resolved — a bar that renders the items itself and offers no anchor of its own. GlassModalSheet.show takes a null morphFrom and presents the way it always did, so the tap still does its job and only the morph is lost.