Better, simpler visibility API.

No need to polyfill for Gecko anymore.

lib/polyfill.coffee

@@ -3,20 +3,10 @@ Polyfill =
     Polyfill.visibility()
   visibility: ->
     # page visibility API
-    return if 'visibilityState' of document
-    if 'webkitVisibilityState' of document
-      prefix = 'webkit'
-    else if 'mozVisibilityState' of document
-      prefix = 'moz'
-    else
-      return
-
-    property = prefix + 'VisibilityState'
-    event    = prefix + 'visibilitychange'
-
-    d.visibilityState = d[property]
-    d.hidden = d.visibilityState is 'hidden'
-    $.on d, event, ->
-      d.visibilityState = d[property]
-      d.hidden = d.visibilityState is 'hidden'
-      $.event 'visibilitychange'
+    return unless 'webkitHidden' of document
+    Object.defineProperties HTMLDocument.prototype,
+      visibilityState:
+        get: -> @webkitVisibilityState
+      hidden:
+        get: -> @webkitHidden
+    $.on d, 'webkitvisibilitychange', -> $.event 'visibilitychange'