I'm not sure why you added a callback there: do we have any other (recent) API with the same signature? It seems confusing to me, and the same result could be obtained from the dev using `sidebar.once` and then `sidebar.show`.
I would remove it, if you want to have it such functionality, I would make it at least a `promise` instead of passing a callback:
``js
sidebar.show().then(…)
```
Notice that if we remove the callback from the method signature, we can always add a promise later, if we don't have time to add it now, it will be backward compatible (however, adding a promise, should really be quick).