--- a/widget/InputData.cpp
+++ b/widget/InputData.cpp
@@ -591,31 +591,50 @@ PanGestureInput::UserMultipliedLocalPanD
 
 PinchGestureInput::PinchGestureInput()
   : InputData(PINCHGESTURE_INPUT)
 {
 }
 
 PinchGestureInput::PinchGestureInput(PinchGestureType aType, uint32_t aTime,
                                      TimeStamp aTimeStamp,
+                                     const ScreenPoint& aFocusPoint,
+                                     ParentLayerCoord aCurrentSpan,
+                                     ParentLayerCoord aPreviousSpan,
+                                     Modifiers aModifiers)
+  : InputData(PINCHGESTURE_INPUT, aTime, aTimeStamp, aModifiers)
+  , mType(aType)
+  , mFocusPoint(aFocusPoint)
+  , mCurrentSpan(aCurrentSpan)
+  , mPreviousSpan(aPreviousSpan)
+{
+}
+
+PinchGestureInput::PinchGestureInput(PinchGestureType aType, uint32_t aTime,
+                                     TimeStamp aTimeStamp,
                                      const ParentLayerPoint& aLocalFocusPoint,
                                      ParentLayerCoord aCurrentSpan,
                                      ParentLayerCoord aPreviousSpan,
                                      Modifiers aModifiers)
   : InputData(PINCHGESTURE_INPUT, aTime, aTimeStamp, aModifiers)
   , mType(aType)
   , mLocalFocusPoint(aLocalFocusPoint)
   , mCurrentSpan(aCurrentSpan)
   , mPreviousSpan(aPreviousSpan)
 {
 }
 
 bool
 PinchGestureInput::TransformToLocal(const ScreenToParentLayerMatrix4x4& aTransform)
-{ 
+{
+  if (mFocusPoint == BothFingersLifted<ScreenPixel>()) {
+    // Special value, no transform required.
+    mLocalFocusPoint = BothFingersLifted();
+    return true;
+  }
   Maybe<ParentLayerPoint> point = UntransformBy(aTransform, mFocusPoint);
   if (!point) {
     return false;
   }
   mLocalFocusPoint = *point;
   return true;
 }
 

--- a/widget/InputData.h
+++ b/widget/InputData.h
@@ -413,16 +413,28 @@ protected:
 public:
   MOZ_DEFINE_ENUM_AT_CLASS_SCOPE(
     PinchGestureType, (
       PINCHGESTURE_START,
       PINCHGESTURE_SCALE,
       PINCHGESTURE_END
   ));
 
+  // Construct a pinch gesture from a Screen point.
+  // (Technically, we should take the span values in Screen pixels as well,
+  // but that would require also storing them in Screen pixels and then
+  // converting them in TransformToLocal() like the focus point. Since pinch
+  // gesture events are processed by the root content APZC, the only transform
+  // between Screen and ParentLayer pixels should be a translation, which is
+  // irrelevant to span values, so we don't bother.)
+  PinchGestureInput(PinchGestureType aType, uint32_t aTime, TimeStamp aTimeStamp,
+                    const ScreenPoint& aFocusPoint,
+                    ParentLayerCoord aCurrentSpan,
+                    ParentLayerCoord aPreviousSpan, Modifiers aModifiers);
+
   // Construct a pinch gesture from a ParentLayer point.
   // mFocusPoint remains (0,0) unless it's set later.
   PinchGestureInput(PinchGestureType aType, uint32_t aTime, TimeStamp aTimeStamp,
                     const ParentLayerPoint& aLocalFocusPoint,
                     ParentLayerCoord aCurrentSpan,
                     ParentLayerCoord aPreviousSpan, Modifiers aModifiers);
 
   bool TransformToLocal(const ScreenToParentLayerMatrix4x4& aTransform);