Re: [whatwg/dom] Add an optional options dictionary to `closest` to allow jumping across shadow boundaries (Issue #1265)

GarrettS left a comment (whatwg/dom#1265)

It doesn't do a composed walk on its own. It does a composed walk because that is how the new API signature gets defined. And if you can't get an ownerDocument or anything, pass it `null` to signal "keep goin thru shadow root"; an imperative override: "search until node X".

As for the workaround, I think I see where you're coming from now: if `target.closest('#STOP-HERE, button')` hits, it's in the direct ancestral path between target, up to and including that boundary, so if it is the boundary, `#STOP-HERE`, that guarantees there is no button on the path.

Let's say best case, you have a good selector for the ancestor say `.boundary` and don't need to stamp it with `#STOP-HERE`. How's it hold up?
```js
const match = target.closest('.boundary, button');
const finalMatch = (match && !match.matches('.boundary')) ? match : null;
```
That could work. Clunks, tho. Extra conditional checks in the code. Selector engine evaluates both branches of that string at every step of the ancestral climb, then `matches` to check one more time what matched. Computationally heavier than an internal pointer-equality check (currentNode === ancestor). Plus, it requires having a good selector for the boundary node, which isn't always the case.

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