Do not generate worker events when not visible

js/detector.js

@@ -13,6 +13,8 @@ var detector =
         list: [],
     },
 
+    visible: true,
+
     width: 400,
     height: 400,
 
@@ -268,6 +270,10 @@ var detector =
 
     addEventExternal: function()
     {
+        if (!detector.visible) {
+            return;
+        }
+
         var num = Math.min(30 * achievements.count.workers / 10, 30);
 
         for (var i = 0; i < num; i++) {

js/ui.js

@@ -7,3 +7,37 @@ $(function() {
     $('.scrollable').height(h + 'px');
   });
 });
+
+(function() {
+    var hidden = "hidden";
+
+    // Standards:
+    if (hidden in document)
+        document.addEventListener("visibilitychange", onchange);
+    else if ((hidden = "mozHidden") in document)
+        document.addEventListener("mozvisibilitychange", onchange);
+    else if ((hidden = "webkitHidden") in document)
+        document.addEventListener("webkitvisibilitychange", onchange);
+    else if ((hidden = "msHidden") in document)
+        document.addEventListener("msvisibilitychange", onchange);
+    // IE 9 and lower:
+    else if ('onfocusin' in document)
+        document.onfocusin = document.onfocusout = onchange;
+    // All others:
+    else
+        window.onpageshow = window.onpagehide 
+            = window.onfocus = window.onblur = onchange;
+
+    function onchange (evt) {
+        var v = 'visible', h = 'hidden',
+            evtMap = { 
+                focus:v, focusin:v, pageshow:v, blur:h, focusout:h, pagehide:h 
+            };
+
+        evt = evt || window.event;
+        if (evt.type in evtMap)
+            detector.visible = evtMap[evt.type] == 'visible';
+        else        
+            detector.visible = !this[hidden];
+    }
+})();