Event.composed

The read-only composed property of the Event interface returns a boolean value which indicates whether or not the event will propagate across the shadow DOM boundary into the standard DOM.

All UA-dispatched UI events are composed (click/touch/mouseover/copy/paste, etc.). Most other types of events are not composed, and so will return false. For example, this includes synthetic events that are created without their composed option set to true.

Propagation only occurs if the bubbles property is also true. However, capturing only composed events are also handled at host as if they were in AT_TARGET phase. You can determine the path the event will follow through the shadow root to the DOM root by calling composedPath().