Happy DOM Event Handling

Duplicate event handler invocations in Happy DOM are typically caused by listener accumulation—registering the same listener multiple times—rather than a framework defect or shadow DOM event propagation issue.

Happy DOM correctly enforces shadow DOM event boundaries, so events propagate across shadow boundaries as specified by the standard; unexpected duplicate calls usually stem from duplicate listener registration, not boundary enforcement failures.

You should avoid adding re-entrancy guards to mask duplicate event handler calls; the root cause is almost always listener accumulation from registering the same listener more than once, which should be fixed at the registration site.

Debug duplicate event listener invocations by inspecting where and how often listeners are registered on your web component; confirm that listener accumulation during setup is the root cause before considering any event propagation or shadow DOM concerns.

Yes, Happy DOM correctly handles shadow DOM event dispatching and enforces event boundaries as specified; when handlers fire multiple times, the issue is typically duplicate listener registration within your test or component setup, not a shadow DOM propagation defect.