diff --git a/widget/BasicEvents.h b/widget/BasicEvents.h
index 97ea284e1578..cb3bab111f24 100644
--- a/widget/BasicEvents.h
+++ b/widget/BasicEvents.h
@@ -264,6 +264,59 @@ public:
 
 class WidgetEvent : public WidgetEventTime
 {
+private:
+  void SetDefaultCancelableAndBubbles()
+  {
+    switch (mClass) {
+      case eEditorInputEventClass:
+        mFlags.mCancelable = false;
+        mFlags.mBubbles = mFlags.mIsTrusted;
+        break;
+      case eMouseEventClass:
+        mFlags.mCancelable = (mMessage != eMouseEnter &&
+                              mMessage != eMouseLeave);
+        mFlags.mBubbles = (mMessage != eMouseEnter &&
+                           mMessage != eMouseLeave);
+        break;
+      case ePointerEventClass:
+        mFlags.mCancelable = (mMessage != ePointerEnter &&
+                              mMessage != ePointerLeave &&
+                              mMessage != ePointerCancel &&
+                              mMessage != ePointerGotCapture &&
+                              mMessage != ePointerLostCapture);
+        mFlags.mBubbles = (mMessage != ePointerEnter &&
+                           mMessage != ePointerLeave);
+        break;
+      case eDragEventClass:
+        mFlags.mCancelable = (mMessage != eDragExit &&
+                              mMessage != eDragLeave &&
+                              mMessage != eDragEnd);
+        mFlags.mBubbles = true;
+        break;
+      case eSMILTimeEventClass:
+        mFlags.mCancelable = false;
+        mFlags.mBubbles = false;
+        break;
+      case eTransitionEventClass:
+      case eAnimationEventClass:
+      case eSVGZoomEventClass:
+        mFlags.mCancelable = false;
+        mFlags.mBubbles = true;
+        break;
+      case eCompositionEventClass:
+        // XXX compositionstart is cancelable in draft of DOM3 Events.
+        //     However, it doesn't make sense for us, we cannot cancel
+        //     composition when we send compositionstart event.
+        mFlags.mCancelable = false;
+        mFlags.mBubbles = true;
+        break;
+      default:
+        mFlags.mCancelable = true;
+        mFlags.mBubbles = true;
+        break;
+    }
+  }
+
 protected:
   WidgetEvent(bool aIsTrusted,
               EventMessage aMessage,
@@ -278,8 +331,7 @@ protected:
     MOZ_COUNT_CTOR(WidgetEvent);
     mFlags.Clear();
     mFlags.mIsTrusted = aIsTrusted;
-    mFlags.mCancelable = true;
-    mFlags.mBubbles = true;
+    SetDefaultCancelableAndBubbles();
   }
 
   WidgetEvent()
@@ -290,18 +342,8 @@ protected:
 
 public:
   WidgetEvent(bool aIsTrusted, EventMessage aMessage)
-    : WidgetEventTime()
-    , mClass(eBasicEventClass)
-    , mMessage(aMessage)
-    , mRefPoint(0, 0)
-    , mLastRefPoint(0, 0)
-    , mSpecifiedEventType(nullptr)
+    : WidgetEvent(aIsTrusted, aMessage, eBasicEventClass)
   {
-    MOZ_COUNT_CTOR(WidgetEvent);
-    mFlags.Clear();
-    mFlags.mIsTrusted = aIsTrusted;
-    mFlags.mCancelable = true;
-    mFlags.mBubbles = true;
   }
 
   virtual ~WidgetEvent()
diff --git a/widget/ContentEvents.h b/widget/ContentEvents.h
index fe979c7c73e4..fc83aaa057c9 100644
--- a/widget/ContentEvents.h
+++ b/widget/ContentEvents.h
@@ -244,7 +244,6 @@ public:
     : WidgetEvent(aIsTrusted, aMessage, eTransitionEventClass)
     , mElapsedTime(0.0)
   {
-    mFlags.mCancelable = false;
   }
 
   virtual WidgetEvent* Duplicate() const override
@@ -289,7 +288,6 @@ public:
     : WidgetEvent(aIsTrusted, aMessage, eAnimationEventClass)
     , mElapsedTime(0.0)
   {
-    mFlags.mCancelable = false;
   }
 
   virtual WidgetEvent* Duplicate() const override
@@ -330,7 +328,6 @@ public:
   InternalSVGZoomEvent(bool aIsTrusted, EventMessage aMessage)
     : WidgetGUIEvent(aIsTrusted, aMessage, nullptr, eSVGZoomEventClass)
   {
-    mFlags.mCancelable = false;
   }
 
   virtual WidgetEvent* Duplicate() const override
@@ -366,8 +363,6 @@ public:
   InternalSMILTimeEvent(bool aIsTrusted, EventMessage aMessage)
     : InternalUIEvent(aIsTrusted, aMessage, eSMILTimeEventClass)
   {
-    mFlags.mBubbles = false;
-    mFlags.mCancelable = false;
   }
 
   virtual WidgetEvent* Duplicate() const override
diff --git a/widget/MouseEvents.h b/widget/MouseEvents.h
index d3b580432475..b96a57254eb0 100644
--- a/widget/MouseEvents.h
+++ b/widget/MouseEvents.h
@@ -217,15 +217,6 @@ protected:
     , exit(eChild)
     , clickCount(0)
   {
-    switch (aMessage) {
-      case eMouseEnter:
-      case eMouseLeave:
-        mFlags.mBubbles = false;
-        mFlags.mCancelable = false;
-        break;
-      default:
-        break;
-    }
   }
 
 public:
@@ -237,17 +228,8 @@ public:
     acceptActivation(false), ignoreRootScrollFrame(false),
     reason(aReason), context(aContext), exit(eChild), clickCount(0)
   {
-    switch (aMessage) {
-      case eMouseEnter:
-      case eMouseLeave:
-        mFlags.mBubbles = false;
-        mFlags.mCancelable = false;
-        break;
-      case eContextMenu:
-        button = (context == eNormal) ? eRightButton : eLeftButton;
-        break;
-      default:
-        break;
+    if (aMessage == eContextMenu) {
+      button = (context == eNormal) ? eRightButton : eLeftButton;
     }
   }
 
@@ -337,8 +319,6 @@ public:
     , mUserCancelled(false)
     , mDefaultPreventedOnContent(false)
   {
-    mFlags.mCancelable =
-      (aMessage != eDragExit && aMessage != eDragLeave && aMessage != eDragEnd);
   }
 
   virtual WidgetEvent* Duplicate() const override
@@ -675,7 +655,6 @@ public:
     , height(0)
     , isPrimary(true)
   {
-    UpdateFlags();
   }
 
   explicit WidgetPointerEvent(const WidgetMouseEvent& aEvent)
@@ -685,25 +664,6 @@ public:
     , isPrimary(true)
   {
     mClass = ePointerEventClass;
-    UpdateFlags();
-  }
-
-  void UpdateFlags()
-  {
-    switch (mMessage) {
-      case ePointerEnter:
-      case ePointerLeave:
-        mFlags.mBubbles = false;
-        mFlags.mCancelable = false;
-        break;
-      case ePointerCancel:
-      case ePointerGotCapture:
-      case ePointerLostCapture:
-        mFlags.mCancelable = false;
-        break;
-      default:
-        break;
-    }
   }
 
   virtual WidgetEvent* Duplicate() const override
diff --git a/widget/TextEvents.h b/widget/TextEvents.h
index db54f757c688..42fe75eb904b 100644
--- a/widget/TextEvents.h
+++ b/widget/TextEvents.h
@@ -498,10 +498,6 @@ public:
     , mNativeIMEContext(aWidget)
     , mOriginalMessage(eVoidEvent)
   {
-    // XXX compositionstart is cancelable in draft of DOM3 Events.
-    //     However, it doesn't make sense for us, we cannot cancel composition
-    //     when we send compositionstart event.
-    mFlags.mCancelable = false;
   }
 
   virtual WidgetEvent* Duplicate() const override
@@ -854,14 +850,6 @@ public:
     : InternalUIEvent(aIsTrusted, aMessage, aWidget, eEditorInputEventClass)
     , mIsComposing(false)
   {
-    if (!aIsTrusted) {
-      mFlags.mBubbles = false;
-      mFlags.mCancelable = false;
-      return;
-    }
-
-    mFlags.mBubbles = true;
-    mFlags.mCancelable = false;
   }
 
   virtual WidgetEvent* Duplicate() const override