diff --git a/content/events/src/nsDOMMouseEvent.cpp b/content/events/src/nsDOMMouseEvent.cpp
index 75d5aa758739..8ba574007855 100644
--- a/content/events/src/nsDOMMouseEvent.cpp
+++ b/content/events/src/nsDOMMouseEvent.cpp
@@ -54,6 +54,9 @@ nsDOMMouseEvent::nsDOMMouseEvent(nsPresContext* aPresContext,
   // DOM event.
   
   if (aEvent) {
+    NS_ASSERTION(static_cast<nsMouseEvent*>(mEvent)->reason
+                 != nsMouseEvent::eSynthesized,
+                 "Don't dispatch DOM events from synthesized mouse events");
     mEventIsInternal = PR_FALSE;
   }
   else {
diff --git a/content/events/src/nsEventStateManager.cpp b/content/events/src/nsEventStateManager.cpp
index 264f2f4e3b7c..51e53b39d516 100644
--- a/content/events/src/nsEventStateManager.cpp
+++ b/content/events/src/nsEventStateManager.cpp
@@ -184,6 +184,15 @@ static PRUint32 gPixelScrollDeltaTimeout = 0;
 static nscoord
 GetScrollableLineHeight(nsIFrame* aTargetFrame);
 
+static inline PRBool
+IsMouseEventReal(nsEvent* aEvent)
+{
+  NS_ABORT_IF_FALSE(aEvent->eventStructType == NS_MOUSE_EVENT,
+                    "Not a mouse event");
+  // Return true if not synthesized.
+  return static_cast<nsMouseEvent*>(aEvent)->reason == nsMouseEvent::eReal;
+}
+
 #ifdef DEBUG_DOCSHELL_FOCUS
 static void
 PrintDocTree(nsIDocShellTreeItem* aParentItem, int aLevel)
@@ -509,7 +518,7 @@ nsMouseWheelTransaction::OnEvent(nsEvent* aEvent)
       return;
     case NS_MOUSE_MOVE:
     case NS_DRAGDROP_OVER:
-      if (((nsMouseEvent*)aEvent)->reason == nsMouseEvent::eReal) {
+      if (IsMouseEventReal(aEvent)) {
         // If the cursor is moving to be outside the frame,
         // terminate the scrollwheel transaction.
         nsIntPoint pt = GetScreenPoint((nsGUIEvent*)aEvent);
@@ -1031,7 +1040,7 @@ nsEventStateManager::PreHandleEvent(nsPresContext* aPresContext,
   // when user is not active doesn't change the state to active.
   if (NS_IS_TRUSTED_EVENT(aEvent) &&
       ((aEvent->eventStructType == NS_MOUSE_EVENT  &&
-        static_cast<nsMouseEvent*>(aEvent)->reason == nsMouseEvent::eReal &&
+        IsMouseEventReal(aEvent) &&
         aEvent->message != NS_MOUSE_ENTER &&
         aEvent->message != NS_MOUSE_EXIT) ||
        aEvent->eventStructType == NS_MOUSE_SCROLL_EVENT ||
@@ -2819,13 +2828,15 @@ nsEventStateManager::PostHandleEvent(nsPresContext* aPresContext,
   case NS_MOUSE_BUTTON_UP:
     {
       SetContentState(nsnull, NS_EVENT_STATE_ACTIVE);
-      if (!mCurrentTarget) {
-        nsIFrame* targ;
-        GetEventTarget(&targ);
-      }
-      if (mCurrentTarget) {
-        ret =
-          CheckForAndDispatchClick(presContext, (nsMouseEvent*)aEvent, aStatus);
+      if (IsMouseEventReal(aEvent)) {
+        if (!mCurrentTarget) {
+          nsIFrame* targ;
+          GetEventTarget(&targ);
+        }
+        if (mCurrentTarget) {
+          ret = CheckForAndDispatchClick(presContext, (nsMouseEvent*)aEvent,
+                                         aStatus);
+        }
       }
 
       nsIPresShell *shell = presContext->GetPresShell();
diff --git a/layout/base/nsPresShell.cpp b/layout/base/nsPresShell.cpp
index 765010edbba9..b1b6c94eaee9 100644
--- a/layout/base/nsPresShell.cpp
+++ b/layout/base/nsPresShell.cpp
@@ -6330,8 +6330,8 @@ PresShell::HandleEventWithTarget(nsEvent* aEvent, nsIFrame* aFrame,
   return NS_OK;
 }
 
-inline PRBool
-IsSynthesizedMouseMove(nsEvent* aEvent)
+static inline PRBool
+IsSynthesizedMouseEvent(nsEvent* aEvent)
 {
   return aEvent->eventStructType == NS_MOUSE_EVENT &&
          static_cast<nsMouseEvent*>(aEvent)->reason != nsMouseEvent::eReal;
@@ -6458,7 +6458,9 @@ PresShell::HandleEventInternal(nsEvent* aEvent, nsIView *aView,
     if (GetCurrentEventFrame() && NS_SUCCEEDED(rv)) {
       // We want synthesized mouse moves to cause mouseover and mouseout
       // DOM events (PreHandleEvent above), but not mousemove DOM events.
-      if (!IsSynthesizedMouseMove(aEvent)) {
+      // Synthesized button up events also do not cause DOM events
+      // because they do not have a reliable refPoint.
+      if (!IsSynthesizedMouseEvent(aEvent)) {
         nsPresShellEventCB eventCB(this);
         if (mCurrentEventContent) {
           nsEventDispatcher::Dispatch(mCurrentEventContent, mPresContext,
diff --git a/view/src/nsViewManager.cpp b/view/src/nsViewManager.cpp
index 96b623eeaf54..09a6b0883b5f 100644
--- a/view/src/nsViewManager.cpp
+++ b/view/src/nsViewManager.cpp
@@ -948,7 +948,7 @@ NS_IMETHODIMP nsViewManager::DispatchEvent(nsGUIEvent *aEvent,
     default:
       {
         if ((NS_IS_MOUSE_EVENT(aEvent) &&
-             // Ignore moves that we synthesize.
+             // Ignore mouse events that we synthesize.
              static_cast<nsMouseEvent*>(aEvent)->reason ==
                nsMouseEvent::eReal &&
              // Ignore mouse exit and enter (we'll get moves if the user