autoExpandDelay property
How long the pointer must dwell on an into target that is
collapsed (and has live children to reveal) before the target
auto-expands, the conventional hover-to-open tree affordance.
null or Duration.zero disables auto-expand. The dwell re-arms
on every target change and is cancelled by session end.
Mutable at runtime; captured per drag session, see autoScrollEdgeZone.
Implementation
Duration? autoExpandDelay;