Issue #2135 - Bug 1413102: Ensure Shadow DOM boundaries are dealt properly in event handling

* RE: BasicEvents.h - our WidgetEvent is not movable (yet), so the change that requires that wasn't included.
* Parts of this use code that was introduced in bug 1427511. For now, they were replaced with their equivalents.

dom/events/EventDispatcher.h

@@ -113,7 +113,8 @@ public:
                        WidgetEvent* aEvent,
                        nsIDOMEvent* aDOMEvent,
                        nsEventStatus aEventStatus,
-                       bool aIsInAnon)
+                       bool aIsInAnon,
+                       dom::EventTarget* aTargetInKnownToBeHandledScope)
     : EventChainVisitor(aPresContext, aEvent, aDOMEvent, aEventStatus)
     , mCanHandle(true)
     , mAutomaticChromeDispatch(true)
@@ -126,8 +127,11 @@ public:
     , mRootOfClosedTree(false)
     , mParentIsSlotInClosedTree(false)
     , mParentIsChromeHandler(false)
+    , mRelatedTargetRetargetedInCurrentScope(false)
     , mParentTarget(nullptr)
     , mEventTargetAtParent(nullptr)
+    , mRetargetedRelatedTarget(nullptr)
+    , mTargetInKnownToBeHandledScope(aTargetInKnownToBeHandledScope)
   {
   }
 
@@ -144,8 +148,12 @@ public:
     mRootOfClosedTree = false;
     mParentIsSlotInClosedTree = false;
     mParentIsChromeHandler = false;
+    // Note, we don't clear mRelatedTargetRetargetedInCurrentScope explicitly,
+    // since it is used during event path creation to indicate whether
+    // relatedTarget may need to be retargeted.
     mParentTarget = nullptr;
     mEventTargetAtParent = nullptr;
+    mRetargetedRelatedTarget = nullptr;
   }
 
   dom::EventTarget* GetParentTarget()
@@ -161,6 +169,13 @@ public:
     }
   }
 
+  void IgnoreCurrentTarget()
+  {
+    mCanHandle = false;
+    SetParentTarget(nullptr, false);
+    mEventTargetAtParent = nullptr;
+  }
+
   /**
    * Member that must be set in GetEventTargetParent by event targets. If set to
    * false, indicates that this event target will not be handling the event and
@@ -229,6 +244,13 @@ public:
    */
   bool mParentIsChromeHandler;
 
+  /**
+   * True if event's related target has been already retargeted in the
+   * current 'scope'. This should be set to false initially and whenever
+   * event path creation crosses shadow boundary.
+   */
+  bool mRelatedTargetRetargetedInCurrentScope;
+
 private:
   /**
    * Parent item in the event target chain.
@@ -241,6 +263,19 @@ public:
    * which should be used when the event is handled at mParentTarget.
    */
   dom::EventTarget* mEventTargetAtParent;
+
+  /**
+   * If the related target of the event needs to be retargeted, set this
+   * to a new EventTarget.
+   */
+  dom::EventTarget* mRetargetedRelatedTarget;
+
+  /**
+   * Set to the value of mEvent->mTarget of the previous scope in case of
+   * Shadow DOM or such, and if there is no anonymous content this just points
+   * to the initial target.
+   */
+  dom::EventTarget* mTargetInKnownToBeHandledScope;
 };
 
 class EventChainPostVisitor : public mozilla::EventChainVisitor