Re: [whatwg/dom] [DRAFT] Propagate events into the event's source's tree where appropriate. (PR #1377)

keithamus left a comment (whatwg/dom#1377)

This concept of re-despatching on the host might have confusing consequences for developers, who will be able to observe events pointed at a target that would commonly dispatch those events, but weren't the source of _this_ event. For example:

```html
<button id=b>Hover me</button>
<div id=popoverA popover>
  <template shadowrootmode=open>
    <div id=popoverB popover>
    </div>
  </template>
</div>
<script>
let popoverB = popoverA.shadowRoot.querySelector('#popoverB');
b.addEventListener('mouseover', () => {
  popoverB.showPopover({source:b});
});
b.addEventListener('mouseout', () => {
  popoverB.hidePopover({source:b});
});
</script>
```

AIUI, In this example, when popoverB is shown it will dispatch a `beforetoggle` (and `toggle`) event on `popoverB`, with a source of `b`. The event will then _also_ dispatch on `popoverA`, despite the fact that `popoverA` isn't being shown. Further to that, the event being dispatched on `popoverA` would be otherwise identical to an event that `popoverA` might dispatch - for example the `target`, `source`, `isTrusted`, `composed` fields etc would all be the same.

This feels like it could make it extremely difficult for developers to ascertain when an event has been dispatched because a host element has/is changed/changing state, or whether one of its shadow descendants happened to do change state and the `source` element is causing it to re-despatch.

I think for this to move forward it would probably need some kind of additional property on Events, like `.wasOriginallyDespatchedInTheTargetsShadowButNowIsBeingRedespatchedInThisTreeBecauseYourSourceElementMightWantToKnowAboutIt`. Though I'm not sold on the name; maybe it can be workshopped.

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