This will be cleaner like so:
```
this.dash.connect('menu-closed', Lang.bind(this, function() {
    this._box.sync_hover();
}));
````

Or if you want shorter lines this can became: 
```
this.dash.connect('menu-closed',
    Lang.bind(this, function() {
        this._box.sync_hover();
    })
);
````

_Sidenote_: Naming your anonymous functions can help with testing and debugging.

For the time being, don't worry about this.