--- a/dom/ipc/TabParent.cpp
+++ b/dom/ipc/TabParent.cpp
@@ -42,16 +42,17 @@
 #include "mozilla/UniquePtr.h"
 #include "mozilla/Unused.h"
 #include "nsCOMPtr.h"
 #include "nsContentAreaDragDrop.h"
 #include "nsContentUtils.h"
 #include "nsDebug.h"
 #include "nsFocusManager.h"
 #include "nsFrameLoader.h"
+#include "nsFrameManager.h"
 #include "nsIBaseWindow.h"
 #include "nsIBrowser.h"
 #include "nsIContent.h"
 #include "nsIDocShell.h"
 #include "nsIDocShellTreeOwner.h"
 #include "nsIDOMElement.h"
 #include "nsIDOMEvent.h"
 #include "nsIDOMWindow.h"
@@ -1974,27 +1975,47 @@ TabParent::GetChildProcessOffset()
   if (!frameLoader) {
     return offset;
   }
   nsIFrame* targetFrame = frameLoader->GetPrimaryFrameOfOwningContent();
   if (!targetFrame) {
     return offset;
   }
 
-  // Find out how far we're offset from the nearest widget.
   nsCOMPtr<nsIWidget> widget = GetWidget();
   if (!widget) {
     return offset;
   }
-  nsPoint pt = nsLayoutUtils::GetEventCoordinatesRelativeTo(widget,
-                                                            LayoutDeviceIntPoint(0, 0),
-                                                            targetFrame);
-
-  return LayoutDeviceIntPoint::FromAppUnitsToNearest(
-           pt, targetFrame->PresContext()->AppUnitsPerDevPixel());
+
+  nsPresContext* presContext = targetFrame->PresContext();
+  nsIFrame* rootFrame = presContext->PresShell()->FrameManager()->GetRootFrame();
+  nsView* rootView = rootFrame->GetView();
+  if (!rootView) {
+    return offset;
+  }
+
+  // Note that we don't want to take into account transforms here:
+#if 0
+  nsPoint pt(0, 0);
+  nsLayoutUtils::TransformPoint(targetFrame, rootFrame, pt);
+#endif
+  // In practice, when transforms are applied to this frameLoader, we currently
+  // get the wrong results whether we take transforms into account here or not.
+  // But applying transforms here gives us the wrong results in all
+  // circumstances when transforms are applied, unless they're purely
+  // translational. It also gives us the wrong results whenever CSS transitions
+  // are used to apply transforms, since the offeets aren't updated as the
+  // transition is animated.
+  //
+  // What we actually need to do is apply the transforms to the coordinates of
+  // any events we send to the child, and reverse them for any screen
+  // coordinates that we retrieve from the child.
+
+  nsPoint pt = targetFrame->GetOffsetTo(rootFrame);
+  return -nsLayoutUtils::TranslateViewToWidget(presContext, rootView, pt, widget);
 }
 
 LayoutDeviceIntPoint
 TabParent::GetClientOffset()
 {
   nsCOMPtr<nsIWidget> widget = GetWidget();
   nsCOMPtr<nsIWidget> docWidget = GetDocWidget();
 

--- a/dom/ipc/TabParent.h
+++ b/dom/ipc/TabParent.h
@@ -544,16 +544,19 @@ public:
   // Returns the top-level widget for our frameloader's document.
   already_AddRefed<nsIWidget> GetDocWidget() const;
 
   const TabId GetTabId() const
   {
     return mTabId;
   }
 
+  // Returns the offset from the origin of our frameloader's nearest widget to
+  // the origin of its layout frame. This offset is used to translate event
+  // coordinates relative to the PuppetWidget origin in the child process.
   LayoutDeviceIntPoint GetChildProcessOffset();
 
   // Returns the offset from the on-screen origin of our top-level window's
   // widget (including window decorations) to the origin of our frameloader's
   // nearest widget. This offset is used to translate coordinates from the
   // PuppetWidget's origin to absolute screen coordinates in the child.
   LayoutDeviceIntPoint GetClientOffset();