This is supposed to work in our tests and documentation:
```js
anotherWindow.activate();
// set
windows.activeWindow == anotherWindow;
```
(taken from https://addons.mozilla.org/en-US/developers/docs/sdk/latest/modules/sdk/windows.html) if it's not, we should update them too.

I don't think `focus(mainWindow)` will works, because `mainWindow` is a `BrowserWindow` (SDK) not a ChromeWindow. In that case, if you still want to wait for the activation, I could add a `windows.once('activate', …)`