diff --git a/dom/events/Event.cpp b/dom/events/Event.cpp
index dcb3f53039da..3fa7025e466e 100644
--- a/dom/events/Event.cpp
+++ b/dom/events/Event.cpp
@@ -529,8 +529,19 @@ CSSIntPoint Event::GetScreenCoords(nsPresContext* aPresContext,
     return CSSIntPoint(aPoint.x, aPoint.y);
   }
 
+  // (Potentially) transform the point from the coordinate space of an
+  // out-of-process iframe to the coordinate space of the native
+  // window. The transform can only be applied to a point whose components
+  // are floating-point values, so convert the integer point first, then
+  // transform, and then round the result back to an integer point.
+  LayoutDevicePoint floatPoint(aPoint);
+  LayoutDevicePoint topLevelPoint =
+      guiEvent->mWidget->WidgetToTopLevelWidgetTransform().TransformPoint(
+          floatPoint);
+  LayoutDeviceIntPoint rounded = RoundedToInt(topLevelPoint);
+
   nsPoint pt = LayoutDevicePixel::ToAppUnits(
-      aPoint,
+      rounded,
       aPresContext->DeviceContext()->AppUnitsPerDevPixelAtUnitFullZoom());
 
   if (PresShell* presShell = aPresContext->GetPresShell()) {
@@ -539,7 +550,7 @@ CSSIntPoint Event::GetScreenCoords(nsPresContext* aPresContext,
   }
 
   pt += LayoutDevicePixel::ToAppUnits(
-      guiEvent->mWidget->WidgetToScreenOffset(),
+      guiEvent->mWidget->TopLevelWidgetToScreenOffset(),
       aPresContext->DeviceContext()->AppUnitsPerDevPixelAtUnitFullZoom());
 
   return CSSPixel::FromAppUnitsRounded(pt);
diff --git a/dom/ipc/PBrowser.ipdl b/dom/ipc/PBrowser.ipdl
index 103fd83895a5..f3a245de44ea 100644
--- a/dom/ipc/PBrowser.ipdl
+++ b/dom/ipc/PBrowser.ipdl
@@ -32,7 +32,7 @@ include "mozilla/layers/LayersMessageUtils.h";
 
 using class IPC::Principal from "mozilla/dom/PermissionMessageUtils.h";
 using moveonly mozilla::gfx::PaintFragment from "mozilla/gfx/CrossProcessPaint.h";
-using mozilla::gfx::Matrix from "mozilla/gfx/Matrix.h";
+using mozilla::gfx::Matrix4x4 from "mozilla/gfx/Matrix.h";
 using mozilla::gfx::SurfaceFormat from "mozilla/gfx/Types.h";
 using mozilla::LayoutDeviceIntPoint from "Units.h";
 using mozilla::LayoutDevicePoint from "Units.h";
@@ -643,6 +643,8 @@ child:
 
     async SizeModeChanged(nsSizeMode sizeMode);
 
+    async ChildToParentMatrix(Matrix4x4 aMatrix);
+
     async ParentActivated(bool aActivated);
 
     async SetKeyboardIndicators(UIStateChangeType showAccelerators,
diff --git a/dom/ipc/TabChild.cpp b/dom/ipc/TabChild.cpp
index e7cebe64d6e7..6552c81d30eb 100644
--- a/dom/ipc/TabChild.cpp
+++ b/dom/ipc/TabChild.cpp
@@ -1216,6 +1216,13 @@ mozilla::ipc::IPCResult TabChild::RecvSizeModeChanged(
   return IPC_OK();
 }
 
+mozilla::ipc::IPCResult TabChild::RecvChildToParentMatrix(
+    const mozilla::gfx::Matrix4x4& aMatrix) {
+  mChildToParentConversionMatrix =
+      Some(LayoutDeviceToLayoutDeviceMatrix4x4::FromUnknownMatrix(aMatrix));
+  return IPC_OK();
+}
+
 bool TabChild::UpdateFrame(const RepaintRequest& aRequest) {
   return TabChildBase::UpdateFrameHandler(aRequest);
 }
@@ -1456,6 +1463,14 @@ void TabChild::ProcessPendingCoalescedMouseDataAndDispatchEvents() {
   }
 }
 
+LayoutDeviceToLayoutDeviceMatrix4x4 TabChild::GetChildToParentConversionMatrix() const {
+  if (mChildToParentConversionMatrix) {
+    return *mChildToParentConversionMatrix;
+  }
+  LayoutDevicePoint offset(GetChromeOffset());
+  return LayoutDeviceToLayoutDeviceMatrix4x4::Translation(offset);
+}
+
 void TabChild::FlushAllCoalescedMouseData() {
   MOZ_ASSERT(mCoalesceMouseMoveEvents);
 
diff --git a/dom/ipc/TabChild.h b/dom/ipc/TabChild.h
index c4ab3b875976..7de7cb1cb7db 100644
--- a/dom/ipc/TabChild.h
+++ b/dom/ipc/TabChild.h
@@ -312,6 +312,9 @@ class TabChild final : public TabChildBase,
   virtual mozilla::ipc::IPCResult RecvSizeModeChanged(
       const nsSizeMode& aSizeMode) override;
 
+  virtual mozilla::ipc::IPCResult RecvChildToParentMatrix(
+      const mozilla::gfx::Matrix4x4& aMatrix) override;
+
   mozilla::ipc::IPCResult RecvActivate();
 
   mozilla::ipc::IPCResult RecvDeactivate();
@@ -637,6 +640,10 @@ class TabChild final : public TabChildBase,
   // The HANDLE object for the widget this TabChild in.
   WindowsHandle WidgetNativeData() { return mWidgetNativeData; }
 
+  // The transform from the coordinate space of this TabChild to the coordinate
+  // space of the native window its TabParent is in.
+  mozilla::LayoutDeviceToLayoutDeviceMatrix4x4 GetChildToParentConversionMatrix() const;
+
   // Prepare to dispatch all coalesced mousemove events. We'll move all data
   // in mCoalescedMouseData to a nsDeque; then we start processing them. We
   // can't fetch the coalesced event one by one and dispatch it because we may
@@ -905,6 +912,8 @@ class TabChild final : public TabChildBase,
 
   WindowsHandle mWidgetNativeData;
 
+  Maybe<LayoutDeviceToLayoutDeviceMatrix4x4> mChildToParentConversionMatrix;
+
   // This state is used to keep track of the current visible tabs (the ones
   // rendering layers). There may be more than one if there are multiple browser
   // windows open, or tabs are being warmed up. There may be none if this
diff --git a/dom/ipc/TabParent.cpp b/dom/ipc/TabParent.cpp
index 51e4bdd6c2ac..7e41d93cf339 100644
--- a/dom/ipc/TabParent.cpp
+++ b/dom/ipc/TabParent.cpp
@@ -2062,6 +2062,10 @@ TabParent::GetChildToParentConversionMatrix() {
 void TabParent::SetChildToParentConversionMatrix(
     const LayoutDeviceToLayoutDeviceMatrix4x4& aMatrix) {
   mChildToParentConversionMatrix = Some(aMatrix);
+  if (mIsDestroyed) {
+    return;
+  }
+  mozilla::Unused << SendChildToParentMatrix(aMatrix.ToUnknownMatrix());
 }
 
 LayoutDeviceIntPoint TabParent::GetChildProcessOffset() {
diff --git a/widget/PuppetWidget.cpp b/widget/PuppetWidget.cpp
index 9b0ddb74e2b4..939aaf01d494 100644
--- a/widget/PuppetWidget.cpp
+++ b/widget/PuppetWidget.cpp
@@ -286,6 +286,15 @@ void PuppetWidget::Invalidate(const LayoutDeviceIntRect& aRect) {
   }
 }
 
+mozilla::LayoutDeviceToLayoutDeviceMatrix4x4
+PuppetWidget::WidgetToTopLevelWidgetTransform() {
+  if (!GetOwningTabChild()) {
+    NS_WARNING("PuppetWidget without Tab does not have transform information.");
+    return mozilla::LayoutDeviceToLayoutDeviceMatrix4x4();
+  }
+  return GetOwningTabChild()->GetChildToParentConversionMatrix();
+}
+
 void PuppetWidget::InitEvent(WidgetGUIEvent& aEvent,
                              LayoutDeviceIntPoint* aPoint) {
   if (nullptr == aPoint) {
diff --git a/widget/PuppetWidget.h b/widget/PuppetWidget.h
index 351fd61dac30..9cda6011849f 100644
--- a/widget/PuppetWidget.h
+++ b/widget/PuppetWidget.h
@@ -129,10 +129,17 @@ class PuppetWidget : public nsBaseWidget,
     return NS_ERROR_UNEXPECTED;
   }
 
+  virtual mozilla::LayoutDeviceToLayoutDeviceMatrix4x4
+  WidgetToTopLevelWidgetTransform() override;
+
   virtual LayoutDeviceIntPoint WidgetToScreenOffset() override {
     return GetWindowPosition() + GetChromeOffset();
   }
 
+  virtual LayoutDeviceIntPoint TopLevelWidgetToScreenOffset() override {
+    return GetWindowPosition();
+  }
+
   int32_t RoundsWidgetCoordinatesTo() override;
 
   void InitEvent(WidgetGUIEvent& aEvent,
diff --git a/widget/nsIWidget.h b/widget/nsIWidget.h
index 7b492353711b..56be84064ac6 100644
--- a/widget/nsIWidget.h
+++ b/widget/nsIWidget.h
@@ -1345,6 +1345,26 @@ class nsIWidget : public nsISupports {
    */
   virtual LayoutDeviceIntPoint WidgetToScreenOffset() = 0;
 
+  /**
+   * The same as WidgetToScreenOffset(), except in the case of
+   * PuppetWidget where this method omits the chrome offset.
+   */
+  virtual LayoutDeviceIntPoint TopLevelWidgetToScreenOffset() {
+    return WidgetToScreenOffset();
+  }
+
+  /**
+   * For a PuppetWidget, returns the transform from the coordinate
+   * space of the PuppetWidget to the coordinate space of the
+   * top-level native widget.
+   *
+   * Identity transform in other cases.
+   */
+  virtual mozilla::LayoutDeviceToLayoutDeviceMatrix4x4
+  WidgetToTopLevelWidgetTransform() {
+    return mozilla::LayoutDeviceToLayoutDeviceMatrix4x4();
+  }
+
   /**
    * Given the specified client size, return the corresponding window size,
    * which includes the area for the borders and titlebar. This method