Library Affected:
workbox-window

Browser & Platform:
All browsers that support service workers and message channels

Issue or Feature Request Description:
When sending a message from the service worker to all clients (in the case of testing, only one client), the client does not receive the message port with which to respond. This happens when adding the message event liste

In Settings : 'Add a new side account' button

Step #2 in Side account creation

(a very light & quick version of onboarding, with only nickname step)
![side account mode copy](https://user-images.githubusercontent.com/37345040/99953009-dd7db900-2d80-11eb-9f6f-e

Here is an issues with detailed description: NekR/offline-plugin#77