Implement pop-up blocking for Safari

It works similarly to the xhr intercepting, except here the window.open
global function is being overridden.
Note that it could only work if the site's Content Security Policy allows
inline scripts, and the script on the webpage doesn't have a copy of the
original window.open function (it can happen only if the page has an
inline script in its head element, where the reference to the original
function can be obtained - likely this cannot be prevented in Safari).

src/js/vapi-client.js

@@ -1,3 +1,4 @@
+/* global addMessageListener, removeMessageListener, sendAsyncMessage */
 // for non background pages
 
 (function() {
@@ -200,7 +201,6 @@ if (window.chrome) {
 
         if (details) {
             details.url = linkHelper.href;
-            details.type = 'xmlhttprequest';
         }
         else {
             details = {
@@ -237,6 +237,11 @@ if (window.chrome) {
                 default:
                     details.type = 'other';
             }
+
+            // This can run even before the first DOMSubtreeModified event fired
+            if (firstMutation) {
+                firstMutation();
+            }
         }
 
         // tabId is determined in the background script
@@ -252,25 +257,22 @@ if (window.chrome) {
             return false;
         }
         // local mirroring, response is a data: URL here
-        else if (typeof response === 'string') {
-            if (details.type === 'script') {
-                e.preventDefault();
-                return response;
-            }
-            else if (details.type === 'script') {
-                e.preventDefault();
-                details = document.createElement('script');
-                details.textContent = atob(response.slice(35));
-                e.target.parentNode.insertBefore(details, e.target);
-                details.parentNode.removeChild(details);
-            }
+        else if (typeof response === 'string' && details.type === 'script') {
+            // Content Security Policy with disallowed inline scripts may break things
+            e.preventDefault();
+            details = document.createElement('script');
+            details.textContent = atob(response.slice(response.indexOf(',', 20) + 1));
+            e.target.parentNode.insertBefore(details, e.target);
+            details.parentNode.removeChild(details);
         }
     };
 
     document.addEventListener('beforeload', onBeforeLoad, true);
 
-    // intercepting xhr requests
-    setTimeout(function() {
+    // blocking pop-ups and intercepting xhr requests
+    var firstMutation = function() {
+        document.removeEventListener('DOMSubtreeModified', firstMutation, true);
+        firstMutation = null;
         var randomEventName = parseInt(Math.random() * 1e15, 10).toString(36);
         var beforeLoadEvent = document.createEvent('Event');
         beforeLoadEvent.initEvent('beforeload');
@@ -278,41 +280,35 @@ if (window.chrome) {
         window.addEventListener(randomEventName, function(e) {
             var result = onBeforeLoad(beforeLoadEvent, e.detail);
 
-            if (onBeforeLoad(beforeLoadEvent, e.detail) === false) {
+            if (result === false) {
                 e.detail.url = false;
             }
-            else if (typeof result === 'string') {
-                e.detail.url = result;
-            }
         }, true);
 
-        // since the extension context is unable to reach the page context
-        var tempScript = document.createElement('script');
-        tempScript.onload = function() {
-            this.parentNode.removeChild(this);
-        };
-        document.head.appendChild(tempScript).src = "data:application/x-javascript;base64," + btoa(["(function() {",
-            "var xhr_open = XMLHttpRequest.prototype.open;",
-
-            "XMLHttpRequest.prototype.open = function(method, url, async, u, p) {",
-                "var ev = document.createEvent('CustomEvent');",
-                "var detail = {url: url};",
-                "ev.initCustomEvent(",
-                    "'" + randomEventName + "',",
-                    "false, false,",
-                    "detail",
+        // the extension context is unable to reach the page context,
+        // also this only works when Content Security Policy allows inline scripts
+        var tmpJS = document.createElement('script');
+        tmpJS.textContent = ["(function() {",
+            "var block = function(u, t) {",
+                "var e = document.createEvent('CustomEvent'),",
+                    "d = {url: u, type: t};",
+                "e.initCustomEvent(",
+                    "'" + randomEventName + "', !1, !1, d",
                 ");",
-                "window.dispatchEvent(ev);",
-                "if (detail.url === false) {",
-                    "throw Error;",
-                "}",
-                "else if (typeof detail.url === 'string') {",
-                    "url = detail.url;",
-                "}",
-                "return xhr_open.call(this, method, url, async, u, p);",
+                "dispatchEvent(e);",
+                "return d.url === !1;",
+            "}, wo = open, xo = XMLHttpRequest.prototype.open;",
+            "open = function(u) {",
+                "return block(u, 'popup') ? null : wo.apply(this, [].slice.call(arguments));",
+            "};",
+            "XMLHttpRequest.prototype.open = function(m, u) {",
+                "return block(u, 'xmlhttprequest') ? null : xo.apply(this, [].slice.call(arguments));",
             "};",
-        "})();"].join(''));
-    }, 0);
+        "})();"].join('');
+        document.head.removeChild(document.head.appendChild(tmpJS));
+    };
+
+    document.addEventListener('DOMSubtreeModified', firstMutation, true);
 
     var onContextMenu = function(e) {
         var details = {