ItemClick doesn't work with stateReducer inside an iframe

- `downshift` version: 6.1.0
- `node` version: codesandbox
- `npm` (or `yarn`) version: codesandbox

**What you did**:
I tried to combine the [default example](https://github.com/downshift-js/downshift/tree/master/src/hooks/useMultipleSelection#usage) of multiple select and [that](https://github.com/downshift-js/downshift/tree/master/src/hooks/useCombobox#statereducer) state reducer part to achieve my dropdown not being closed after item click. I also wrap it in an iframe(using `react-frame-component`), cause I have such setup for my components in the documentation environment.

**What happened**:


ItemClick change type doesn't trigger stateReducer, which causes menu to close, although InputKeyDownEnter does.

**Reproduction repository**:

https://codesandbox.io/s/gallant-meadow-660h4?file=/src/index.js

**Problem description**:
`stateReducer` doesn't fire with `useCombobox.stateChangeTypes.ItemClick` inside an iframe

**Suggested solution**:
I could implement the fix with some advise help. Do you have any idea, what causes this?

Beginne mit der verknüpften CodeSandbox-Reproduktion und vergleiche das Beispiel für Mehrfachauswahl mit dem useCombobox stateReducer-Beispiel, wobei du dich auf useCombobox.stateChangeTypes.ItemClick innerhalb des iframes konzentrierst. Prüfe, warum ItemClick stateReducer nicht erreicht, während InputKeyDownEnter dies tut. Als abgeschlossen gilt die Aufgabe, wenn der Reducer ItemClick im iframe empfängt und das Menü wie konfiguriert geöffnet bleibt.

Vom Indexierungsmodell aus dem Issue-Text verfasst.