- Electron version: 0.37.2
- Operating system: OS X 10.11.3 

https://github.com/atom/electron/pull/4700 added support for the `visibilitychange` event (thank you @zcbenz!), but that event doesn't make its way into `<webview>`s, which are always "visible" regardless of their host page's `document.visibilityState`.

Tested using the following setup

Host page:

``` coffee
console.log "<webview> host page initial visibilityState =", document.visibilityState

document.addEventListener "visibilitychange", ->
  console.log "<webview> host page visibilitychange event, visibilityState =", document.visibilityState
```

`<webview>`:

``` coffee
console.log "<webview> initial visibilityState =", document.visibilityState

addEventListener "visibilitychange", ->
  console.log "<webview> visibilitychange, visibilityState =", document.visibilityState

setInterval ->
  console.log "<webview> visibilityState =", document.visibilityState
, 3000
```

Host page logs:

```
<webview> host page initial visibilityState = visible
<webview> host page visibilitychange event, visibilityState = hidden
<webview> host page visibilitychange event, visibilityState = visible
```

`<webview>` logs:

```
<webview> initial visibilityState = visible
(24) <webview> visibilityState = visible
```

atom/browser/web_view_manager.h
: embedder_process_id(embedder_process_id),
          element_instance_id(element_instance_id) {}
    bool operator<(const ElementInstanceKey& other) const {
      if (embedder_process_id != other.embedder_process_id)
        return embedder_process_id < other.embedder_process_id;
      return element_instance_id < other.element_instance_id;
    }
    bool operator==(const ElementInstanceKey& other) const {
      return (embedder_process_id == other.embedder_process_id) &&
          (element_instance_id == other.element_instance_id);
    }
  };
  
  std::map<ElementInstanceKey, int> element_instance_id_to_guest_map_;
  DISALLOW_COPY_AND_ASSIGN(WebViewManager);
};
}  
#endif