Why is not enough here? I believe in our code we would have:

```js
if (isPrivate(window)) { … }
```

if it's that the case, then for our unit test should be fine too. So why exactly it's not enough here? If it's not enough it seems to me that we want to test the return value of `isPrivate` more than if "the sidebar is open in a new private window". In that case, this such test should exists, but not in this module.

Again, this is a "nit" for this review so it's not a blocker; however I do not see the reason you mentioned here and I'd like to know in order to avoid similar confusion in the future.