Move to using mutation observing to reduce page overhead (#985)

Core/contentblocker.js

@@ -537,6 +537,7 @@ _utf8_encode : function (string) {
 
     // public
     function shouldBlock(trackerUrl, type) {
+        seenUrls.add(trackerUrl)
         let startTime = performance.now()
 
         if (!blockingEnabled) {
@@ -589,14 +590,18 @@ _utf8_encode : function (string) {
         return false
     }
 
+    const seenUrls = new Set()
+    function hasNotSeen(url) {
+        return !seenUrls.has(url)
+    }
+
     function processPage() {
-        [].slice.apply(document.scripts).forEach(function(el) {
+        [...document.scripts].filter(hasNotSeen).forEach((el) => {
             if (shouldBlock(el.src, 'SCRIPT')) {
                 duckduckgoDebugMessaging.log("blocking load")
             }
-
         });
-        [].slice.apply(document.images).forEach(function(el) {
+        [...document.images].filter(hasNotSeen).forEach((el) => {
           // If the image's natural width is zero, then it has not loaded so we
           // can assume that it may have been blocked.
           if (el.naturalWidth === 0) {
@@ -605,25 +610,34 @@ _utf8_encode : function (string) {
               }
           }
         });
-        [].slice.apply(document.querySelectorAll('link')).forEach(function(el) {
+        [...document.querySelectorAll('link')].filter(hasNotSeen).forEach((el) => {
             if (shouldBlock(el.href, 'LINK')) {
                 duckduckgoDebugMessaging.log("blocking load")
             }
         });
-        [].slice.apply(document.querySelectorAll('iframe')).forEach(function(el) {
-          if (shouldBlock(el.src, 'IFRAME')) {
-              duckduckgoDebugMessaging.log("blocking load")
-          }
+        [...document.querySelectorAll('iframe')].filter(hasNotSeen).forEach((el) => {
+            if (shouldBlock(el.src, 'IFRAME')) {
+                duckduckgoDebugMessaging.log("blocking load")
+            }
         });
-        scheduleProcessPage()
     }
-
-    var interval = 1
-    function scheduleProcessPage() {
-        interval *= 2
-        setTimeout(processPage, interval * 1000)
+
+    function debounce(func, wait) {
+        let timeout
+        return function () {
+            clearTimeout(timeout)
+            timeout = setTimeout(() => {
+                func.apply(this, arguments)
+            }, wait)
+        }
     }
 
+    const observer = new MutationObserver(debounce((mutations, o) => {
+        processPage()
+    }, 100))
+    const rootElement = document.body || document.documentElement
+    observer.observe(rootElement, { childList: true, subtree: true });
+
     // Init
     (function() {