Apparently the Rollup bundle for these tests defaults to `es` format, meaning it expects to be loaded at runtime as native ESM. This was not happening because it was loaded as a regular `<script src="...">` tag (note the lack of `type="module"`).
This is problematic because Rollup assumed it would be running in a scoped environment, meaning [this function](adb8d1078d/packages/core/primitives/event-dispatch/src/event.ts (L45)), which happens to be named `addEventListener` but does *not* implement the `EventTarget.prototype.addEventListener` contract, was being bundled as a simple:
```javascript
function addEventListener(element, ...) {
// ...
}
```
Since this was loaded with no `type="module"` or `'use strict';`, the script executed in "sloppy mode", meaning all `var` statements and function definitions are implicitly global. Since `window` *is* the `globalThis` object, this random `addEventListener` function clobbers the actual `window.addEventListener` and breaks any calls to it because they're not implementing the same contract.
Fix is to just use `<script src="..." type="module">`. Alternatively we could bundle in an IIFE, which Rollup does support, but in theory we could depend on external ES modules which aren't bundled, so the `type="module"` seems a little safer and more future-proof.
cf47eb41ff