--- a/content/events/src/nsEventStateManager.cpp
+++ b/content/events/src/nsEventStateManager.cpp
@@ -106,28 +106,30 @@
 
 using namespace mozilla;
 using namespace mozilla::dom;
 
 //#define DEBUG_DOCSHELL_FOCUS
 
 #define NS_USER_INTERACTION_INTERVAL 5000 // ms
 
+static const nsIntPoint kInvalidRefPoint = nsIntPoint(-1,-1);
+
 static bool sLeftClickOnly = true;
 static bool sKeyCausesActivation = true;
 static uint32_t sESMInstanceCount = 0;
 static int32_t sChromeAccessModifier = 0, sContentAccessModifier = 0;
 int32_t nsEventStateManager::sUserInputEventDepth = 0;
 bool nsEventStateManager::sNormalLMouseEventInProcess = false;
 nsEventStateManager* nsEventStateManager::sActiveESM = nullptr;
 nsIDocument* nsEventStateManager::sMouseOverDocument = nullptr;
 nsWeakFrame nsEventStateManager::sLastDragOverFrame = nullptr;
-nsIntPoint nsEventStateManager::sLastRefPoint = nsIntPoint(0,0);
+nsIntPoint nsEventStateManager::sLastRefPoint = kInvalidRefPoint;
 nsIntPoint nsEventStateManager::sLastScreenPoint = nsIntPoint(0,0);
-nsIntPoint nsEventStateManager::sSynthCenteringPoint = nsIntPoint(-1,-1);
+nsIntPoint nsEventStateManager::sSynthCenteringPoint = kInvalidRefPoint;
 nsIntPoint nsEventStateManager::sLastClientPoint = nsIntPoint(0,0);
 bool nsEventStateManager::sIsPointerLocked = false;
 // Reference to the pointer locked element.
 nsWeakPtr nsEventStateManager::sPointerLockedElement;
 // Reference to the document which requested pointer lock.
 nsWeakPtr nsEventStateManager::sPointerLockedDoc;
 nsCOMPtr<nsIContent> nsEventStateManager::sDragOverContent = nullptr;
 
@@ -4157,18 +4159,24 @@ nsEventStateManager::GenerateMouseEnterE
           aEvent->widget->SynthesizeNativeMouseMove(
             center + aEvent->widget->WidgetToScreenOffset());
         } else if (aEvent->refPoint == sSynthCenteringPoint) {
           // This is the "synthetic native" event we dispatched to re-center the
           // pointer. Cancel it so we don't expose the centering move to content.
           aEvent->flags |= NS_EVENT_FLAG_STOP_DISPATCH;
           // Clear sSynthCenteringPoint so we don't cancel other events
           // targeted at the center.
-          sSynthCenteringPoint = nsIntPoint(-1,-1);
+          sSynthCenteringPoint = kInvalidRefPoint;
         }
+      } else if (sLastRefPoint == kInvalidRefPoint) {
+        // We don't have a valid previous mousemove refPoint. This is either
+        // the first move we've encountered, or the mouse has just re-entered
+        // the application window. We should report (0,0) movement for this
+        // case, so make the current and previous refPoints the same.
+        aEvent->lastRefPoint = aEvent->refPoint;
       } else {
         aEvent->lastRefPoint = sLastRefPoint;
       }
 
       // Update the last known refPoint with the current refPoint.
       sLastRefPoint = aEvent->refPoint;
 
       // Get the target content target (mousemove target == mouseover target)
@@ -4192,16 +4200,20 @@ nsEventStateManager::GenerateMouseEnterE
       if (mLastMouseOverFrame &&
           nsContentUtils::GetTopLevelWidget(aEvent->widget) !=
           nsContentUtils::GetTopLevelWidget(mLastMouseOverFrame->GetNearestWidget())) {
         // the MouseOut event widget doesn't have same top widget with
         // mLastMouseOverFrame, it's a spurious event for mLastMouseOverFrame
         break;
       }
 
+      // Reset sLastRefPoint, so that we'll know not to report any
+      // movement the next time we re-enter the window.
+      sLastRefPoint = kInvalidRefPoint;
+
       NotifyMouseOut(aEvent, nullptr);
     }
     break;
   }
 
   // reset mCurretTargetContent to what it was
   mCurrentTargetContent = targetBeforeEvent;
 }