Constant SYNTHETIC_ID_BIT 

pub const SYNTHETIC_ID_BIT: u64 = _; // 9_223_372_036_854_775_808u64

Bit that marks an accesskit::NodeId as belonging to a synthetic node — one with no backing Widget. Widget ids are taffy_id + 1 or (1 << 48) + index and text-run ids sit at bit 62, so nothing else in the tree can reach bit 63 and the space cannot collide.