Constant LISTEN_PREVENT 

pub const LISTEN_PREVENT: u8 = _; // 1u8

Event-modifier bits a listener registration carries (listenFlags, DomOp::Listen) — .prevent / .stop / .once / .capture / .passive, one bit each, in that order. Every host dispatches by them as a browser does (dispatch_path), except .once, which the lowering keeps itself: a host never retires a listener on its own.