Issue 5650 - Prevent attachShadowRoot to create closed shadow root.

inject.preload.js

Index: inject.preload.js
===================================================================
--- a/inject.preload.js
+++ b/inject.preload.js
@@ -133,16 +133,31 @@
           let thisShadow = shadowRoot(this);
           return thisShadow == ourShadowRoot ? null : thisShadow;
         }
       });
     }
   }
 
   /*
+   * Closed shadowRoot prevention.
+   * https://issues.adblockplus.org/ticket/5650
+   */
+  if ("attachShadow" in Element.prototype)
+  {
+    Element.prototype._attachShadow = Element.prototype.attachShadow;
+    Element.prototype.attachShadow = options => {
+      if (options && options.mode == "closed") {
+        options.mode = "open";
+      }
+      return this._attachShadow(options);
+    };
+  }
+
+  /*
    * Shared request checking code, used by both the WebSocket and
    * RTCPeerConnection wrappers.
    */
   let RealCustomEvent = window.CustomEvent;
 
   // If we've been injected into a frame via contentWindow then we can simply
   // grab the copy of checkRequest left for us by the parent document. Otherwise
   // we need to set it up now, along with the event handling functions.