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diff --git a/layout/base/UnitTransforms.h b/layout/base/UnitTransforms.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef MOZ_UNIT_TRANSFORMS_H_
+#define MOZ_UNIT_TRANSFORMS_H_
+
+#include "Units.h"
+#include "mozilla/gfx/Matrix.h"
+#include "mozilla/Maybe.h"
+#include "nsRegion.h"
+
+namespace mozilla {
+
+// Convenience functions for converting an entity from one strongly-typed
+// coordinate system to another without changing the values it stores (this
+// can be thought of as a cast).
+// To use these functions, you must provide a justification for each use!
+// Feel free to add more justifications to PixelCastJustification, along with
+// a comment that explains under what circumstances it is appropriate to use.
+
+enum class PixelCastJustification : uint8_t {
+  // For the root layer, Screen Pixel = Parent Layer Pixel.
+  ScreenIsParentLayerForRoot,
+  // On the layout side, Screen Pixel = LayoutDevice at the outer-window level.
+  LayoutDeviceIsScreenForBounds,
+  // For the root layer, Render Target Pixel = Parent Layer Pixel.
+  RenderTargetIsParentLayerForRoot,
+  // For the root composition size we want to view it as layer pixels in any
+  // layer
+  ParentLayerToLayerForRootComposition,
+  // The Layer coordinate space for one layer is the ParentLayer coordinate
+  // space for its children
+  MovingDownToChildren,
+  // The transform that is usually used to convert between two coordinate
+  // systems is not available (for example, because the object that stores it
+  // is being destroyed), so fall back to the identity.
+  TransformNotAvailable,
+  // When an OS event is initially constructed, its reference point is
+  // technically in screen pixels, as it has not yet accounted for any
+  // asynchronous transforms. This justification is for viewing the initial
+  // reference point as a screen point. The reverse is useful when synthetically
+  // created WidgetEvents need to be converted back to InputData.
+  LayoutDeviceIsScreenForUntransformedEvent,
+  // Similar to LayoutDeviceIsScreenForUntransformedEvent, PBrowser handles
+  // some widget/tab dimension information as the OS does -- in screen units.
+  LayoutDeviceIsScreenForTabDims,
+  // A combination of LayoutDeviceIsScreenForBounds and
+  // ScreenIsParentLayerForRoot, which is how we're using it.
+  LayoutDeviceIsParentLayerForRCDRSF,
+  // Used to treat the product of AsyncTransformComponentMatrix objects
+  // as an AsyncTransformMatrix. See the definitions of these matrices in
+  // LayersTypes.h for details.
+  MultipleAsyncTransforms,
+  // We have reason to believe a layer doesn't have a local transform.
+  // Should only be used if we've already checked or asserted this.
+  NoTransformOnLayer,
+  // LayerPixels are ImagePixels
+  LayerIsImage,
+  // External pixels are the same scale as screen pixels
+  ExternalIsScreen,
+  // LayerToScreenMatrix is used as LayoutDeviceToLayoutDevice, because
+  // out-of-process iframes uses LayoutDevicePixels as the type system-visible
+  // type of their top-level event coordinate space even if technically
+  // inaccurate.
+  ContentProcessIsLayerInUiProcess,
+};
+
+template <class TargetUnits, class SourceUnits>
+gfx::CoordTyped<TargetUnits> ViewAs(const gfx::CoordTyped<SourceUnits>& aCoord,
+                                    PixelCastJustification) {
+  return gfx::CoordTyped<TargetUnits>(aCoord.value);
+}
+template <class TargetUnits, class SourceUnits>
+gfx::IntCoordTyped<TargetUnits> ViewAs(
+    const gfx::IntCoordTyped<SourceUnits>& aCoord, PixelCastJustification) {
+  return gfx::IntCoordTyped<TargetUnits>(aCoord.value);
+}
+template <class TargetUnits, class SourceUnits>
+gfx::SizeTyped<TargetUnits> ViewAs(const gfx::SizeTyped<SourceUnits>& aSize,
+                                   PixelCastJustification) {
+  return gfx::SizeTyped<TargetUnits>(aSize.width, aSize.height);
+}
+template <class TargetUnits, class SourceUnits>
+gfx::IntSizeTyped<TargetUnits> ViewAs(
+    const gfx::IntSizeTyped<SourceUnits>& aSize, PixelCastJustification) {
+  return gfx::IntSizeTyped<TargetUnits>(aSize.width, aSize.height);
+}
+template <class TargetUnits, class SourceUnits>
+gfx::PointTyped<TargetUnits> ViewAs(const gfx::PointTyped<SourceUnits>& aPoint,
+                                    PixelCastJustification) {
+  return gfx::PointTyped<TargetUnits>(aPoint.x, aPoint.y);
+}
+template <class TargetUnits, class SourceUnits>
+gfx::IntPointTyped<TargetUnits> ViewAs(
+    const gfx::IntPointTyped<SourceUnits>& aPoint, PixelCastJustification) {
+  return gfx::IntPointTyped<TargetUnits>(aPoint.x, aPoint.y);
+}
+template <class TargetUnits, class SourceUnits>
+gfx::RectTyped<TargetUnits> ViewAs(const gfx::RectTyped<SourceUnits>& aRect,
+                                   PixelCastJustification) {
+  return gfx::RectTyped<TargetUnits>(aRect.x, aRect.y, aRect.Width(),
+                                     aRect.Height());
+}
+template <class TargetUnits, class SourceUnits>
+gfx::IntRectTyped<TargetUnits> ViewAs(
+    const gfx::IntRectTyped<SourceUnits>& aRect, PixelCastJustification) {
+  return gfx::IntRectTyped<TargetUnits>(aRect.x, aRect.y, aRect.Width(),
+                                        aRect.Height());
+}
+template <class TargetUnits, class SourceUnits>
+gfx::MarginTyped<TargetUnits> ViewAs(
+    const gfx::MarginTyped<SourceUnits>& aMargin, PixelCastJustification) {
+  return gfx::MarginTyped<TargetUnits>(aMargin.top, aMargin.right,
+                                       aMargin.bottom, aMargin.left);
+}
+template <class TargetUnits, class SourceUnits>
+gfx::IntMarginTyped<TargetUnits> ViewAs(
+    const gfx::IntMarginTyped<SourceUnits>& aMargin, PixelCastJustification) {
+  return gfx::IntMarginTyped<TargetUnits>(aMargin.top, aMargin.right,
+                                          aMargin.bottom, aMargin.left);
+}
+template <class TargetUnits, class SourceUnits>
+gfx::IntRegionTyped<TargetUnits> ViewAs(
+    const gfx::IntRegionTyped<SourceUnits>& aRegion, PixelCastJustification) {
+  return gfx::IntRegionTyped<TargetUnits>::FromUnknownRegion(
+      aRegion.ToUnknownRegion());
+}
+template <class NewTargetUnits, class OldTargetUnits, class SourceUnits>
+gfx::ScaleFactor<SourceUnits, NewTargetUnits> ViewTargetAs(
+    const gfx::ScaleFactor<SourceUnits, OldTargetUnits>& aScaleFactor,
+    PixelCastJustification) {
+  return gfx::ScaleFactor<SourceUnits, NewTargetUnits>(aScaleFactor.scale);
+}
+template <class TargetUnits, class SourceUnits>
+Maybe<gfx::IntRectTyped<TargetUnits>> ViewAs(
+    const Maybe<gfx::IntRectTyped<SourceUnits>>& aRect,
+    PixelCastJustification aJustification) {
+  if (aRect.isSome()) {
+    return Some(ViewAs<TargetUnits>(aRect.value(), aJustification));
+  }
+  return Nothing();
+}
+// Unlike the other functions in this category, these functions take the
+// target matrix type, rather than its source and target unit types, as
+// the explicit template argument, so an example invocation is:
+//    ViewAs<ScreenToLayerMatrix4x4>(otherTypedMatrix, justification)
+// The reason is that if it took the source and target unit types as two
+// template arguments, there may be some confusion as to which is the
+// source and which is the target.
+template <class TargetMatrix, class SourceMatrixSourceUnits,
+          class SourceMatrixTargetUnits>
+TargetMatrix ViewAs(const gfx::Matrix4x4Typed<SourceMatrixSourceUnits,
+                                              SourceMatrixTargetUnits>& aMatrix,
+                    PixelCastJustification) {
+  return TargetMatrix::FromUnknownMatrix(aMatrix.ToUnknownMatrix());
+}
+template <class TargetMatrix, class SourceMatrixSourceUnits,
+          class SourceMatrixTargetUnits>
+Maybe<TargetMatrix> ViewAs(
+    const Maybe<gfx::Matrix4x4Typed<SourceMatrixSourceUnits,
+                                    SourceMatrixTargetUnits>>& aMatrix,
+    PixelCastJustification) {
+  if (aMatrix.isSome()) {
+    return Some(TargetMatrix::FromUnknownMatrix(aMatrix->ToUnknownMatrix()));
+  }
+  return Nothing();
+}
+
+// A non-member overload of ToUnknownMatrix() for use on a Maybe<Matrix>.
+// We can't make this a member because we can't inject a member into Maybe.
+template <typename SourceUnits, typename TargetUnits>
+Maybe<gfx::Matrix4x4> ToUnknownMatrix(
+    const Maybe<gfx::Matrix4x4Typed<SourceUnits, TargetUnits>>& aMatrix) {
+  if (aMatrix.isSome()) {
+    return Some(aMatrix->ToUnknownMatrix());
+  }
+  return Nothing();
+}
+
+// Convenience functions for casting untyped entities to typed entities.
+// Using these functions does not require a justification, but once we convert
+// all code to use strongly typed units they should not be needed any longer.
+template <class TargetUnits>
+gfx::PointTyped<TargetUnits> ViewAs(const gfxPoint& aPoint) {
+  return gfx::PointTyped<TargetUnits>(aPoint.x, aPoint.y);
+}
+template <class TargetUnits>
+gfx::PointTyped<TargetUnits> ViewAs(const gfx::Point& aPoint) {
+  return gfx::PointTyped<TargetUnits>(aPoint.x, aPoint.y);
+}
+template <class TargetUnits>
+gfx::RectTyped<TargetUnits> ViewAs(const gfx::Rect& aRect) {
+  return gfx::RectTyped<TargetUnits>(aRect.x, aRect.y, aRect.Width(),
+                                     aRect.Height());
+}
+template <class TargetUnits>
+gfx::IntSizeTyped<TargetUnits> ViewAs(const nsIntSize& aSize) {
+  return gfx::IntSizeTyped<TargetUnits>(aSize.width, aSize.height);
+}
+template <class TargetUnits>
+gfx::IntPointTyped<TargetUnits> ViewAs(const nsIntPoint& aPoint) {
+  return gfx::IntPointTyped<TargetUnits>(aPoint.x, aPoint.y);
+}
+template <class TargetUnits>
+gfx::IntRectTyped<TargetUnits> ViewAs(const nsIntRect& aRect) {
+  return gfx::IntRectTyped<TargetUnits>(aRect.x, aRect.y, aRect.Width(),
+                                        aRect.Height());
+}
+template <class TargetUnits>
+gfx::IntRegionTyped<TargetUnits> ViewAs(const nsIntRegion& aRegion) {
+  return gfx::IntRegionTyped<TargetUnits>::FromUnknownRegion(aRegion);
+}
+// Unlike the other functions in this category, this function takes the
+// target matrix type, rather than its source and target unit types, as
+// the template argument, so an example invocation is:
+//    ViewAs<ScreenToLayerMatrix4x4>(untypedMatrix)
+// The reason is that if it took the source and target unit types as two
+// template arguments, there may be some confusion as to which is the
+// source and which is the target.
+template <class TypedMatrix>
+TypedMatrix ViewAs(const gfx::Matrix4x4& aMatrix) {
+  return TypedMatrix::FromUnknownMatrix(aMatrix);
+}
+
+// Convenience functions for transforming an entity from one strongly-typed
+// coordinate system to another using the provided transformation matrix.
+template <typename TargetUnits, typename SourceUnits>
+static gfx::PointTyped<TargetUnits> TransformBy(
+    const gfx::Matrix4x4Typed<SourceUnits, TargetUnits>& aTransform,
+    const gfx::PointTyped<SourceUnits>& aPoint) {
+  return aTransform.TransformPoint(aPoint);
+}
+template <typename TargetUnits, typename SourceUnits>
+static gfx::IntPointTyped<TargetUnits> TransformBy(
+    const gfx::Matrix4x4Typed<SourceUnits, TargetUnits>& aTransform,
+    const gfx::IntPointTyped<SourceUnits>& aPoint) {
+  return RoundedToInt(
+      TransformBy(aTransform, gfx::PointTyped<SourceUnits>(aPoint)));
+}
+template <typename TargetUnits, typename SourceUnits>
+static gfx::RectTyped<TargetUnits> TransformBy(
+    const gfx::Matrix4x4Typed<SourceUnits, TargetUnits>& aTransform,
+    const gfx::RectTyped<SourceUnits>& aRect) {
+  return aTransform.TransformBounds(aRect);
+}
+template <typename TargetUnits, typename SourceUnits>
+static gfx::IntRectTyped<TargetUnits> TransformBy(
+    const gfx::Matrix4x4Typed<SourceUnits, TargetUnits>& aTransform,
+    const gfx::IntRectTyped<SourceUnits>& aRect) {
+  return RoundedToInt(
+      TransformBy(aTransform, gfx::RectTyped<SourceUnits>(aRect)));
+}
+template <typename TargetUnits, typename SourceUnits>
+static gfx::IntRegionTyped<TargetUnits> TransformBy(
+    const gfx::Matrix4x4Typed<SourceUnits, TargetUnits>& aTransform,
+    const gfx::IntRegionTyped<SourceUnits>& aRegion) {
+  return ViewAs<TargetUnits>(
+      aRegion.ToUnknownRegion().Transform(aTransform.ToUnknownMatrix()));
+}
+
+// Transform |aVector|, which is anchored at |aAnchor|, by the given transform
+// matrix, yielding a point in |TargetUnits|.
+// The anchor is necessary because with 3D tranforms, the location of the
+// vector can affect the result of the transform.
+template <typename TargetUnits, typename SourceUnits>
+static gfx::PointTyped<TargetUnits> TransformVector(
+    const gfx::Matrix4x4Typed<SourceUnits, TargetUnits>& aTransform,
+    const gfx::PointTyped<SourceUnits>& aVector,
+    const gfx::PointTyped<SourceUnits>& aAnchor) {
+  gfx::PointTyped<TargetUnits> transformedStart =
+      TransformBy(aTransform, aAnchor);
+  gfx::PointTyped<TargetUnits> transformedEnd =
+      TransformBy(aTransform, aAnchor + aVector);
+  return transformedEnd - transformedStart;
+}
+
+// UntransformBy() and UntransformVector() are like TransformBy() and
+// TransformVector(), respectively, but are intended for cases where
+// the transformation matrix is the inverse of a 3D projection. When
+// using such transforms, the resulting Point4D is only meaningful
+// if it has a positive w-coordinate. To handle this, these functions
+// return a Maybe object which contains a value if and only if the
+// result is meaningful
+template <typename TargetUnits, typename SourceUnits>
+static Maybe<gfx::PointTyped<TargetUnits>> UntransformBy(
+    const gfx::Matrix4x4Typed<SourceUnits, TargetUnits>& aTransform,
+    const gfx::PointTyped<SourceUnits>& aPoint) {
+  gfx::Point4DTyped<TargetUnits> point = aTransform.ProjectPoint(aPoint);
+  if (!point.HasPositiveWCoord()) {
+    return Nothing();
+  }
+  return Some(point.As2DPoint());
+}
+template <typename TargetUnits, typename SourceUnits>
+static Maybe<gfx::IntPointTyped<TargetUnits>> UntransformBy(
+    const gfx::Matrix4x4Typed<SourceUnits, TargetUnits>& aTransform,
+    const gfx::IntPointTyped<SourceUnits>& aPoint) {
+  gfx::PointTyped<SourceUnits> p = aPoint;
+  gfx::Point4DTyped<TargetUnits> point = aTransform.ProjectPoint(p);
+  if (!point.HasPositiveWCoord()) {
+    return Nothing();
+  }
+  return Some(RoundedToInt(point.As2DPoint()));
+}
+
+// The versions of UntransformBy() that take a rectangle also take a clip,
+// which represents the bounds within which the target must fall. The
+// result of the transform is intersected with this clip, and is considered
+// meaningful if the intersection is not empty.
+template <typename TargetUnits, typename SourceUnits>
+static Maybe<gfx::RectTyped<TargetUnits>> UntransformBy(
+    const gfx::Matrix4x4Typed<SourceUnits, TargetUnits>& aTransform,
+    const gfx::RectTyped<SourceUnits>& aRect,
+    const gfx::RectTyped<TargetUnits>& aClip) {
+  gfx::RectTyped<TargetUnits> rect = aTransform.ProjectRectBounds(aRect, aClip);
+  if (rect.IsEmpty()) {
+    return Nothing();
+  }
+  return Some(rect);
+}
+template <typename TargetUnits, typename SourceUnits>
+static Maybe<gfx::IntRectTyped<TargetUnits>> UntransformBy(
+    const gfx::Matrix4x4Typed<SourceUnits, TargetUnits>& aTransform,
+    const gfx::IntRectTyped<SourceUnits>& aRect,
+    const gfx::IntRectTyped<TargetUnits>& aClip) {
+  gfx::RectTyped<TargetUnits> rect = aTransform.ProjectRectBounds(aRect, aClip);
+  if (rect.IsEmpty()) {
+    return Nothing();
+  }
+  return Some(RoundedToInt(rect));
+}
+
+template <typename TargetUnits, typename SourceUnits>
+static Maybe<gfx::PointTyped<TargetUnits>> UntransformVector(
+    const gfx::Matrix4x4Typed<SourceUnits, TargetUnits>& aTransform,
+    const gfx::PointTyped<SourceUnits>& aVector,
+    const gfx::PointTyped<SourceUnits>& aAnchor) {
+  gfx::Point4DTyped<TargetUnits> projectedAnchor =
+      aTransform.ProjectPoint(aAnchor);
+  gfx::Point4DTyped<TargetUnits> projectedTarget =
+      aTransform.ProjectPoint(aAnchor + aVector);
+  if (!projectedAnchor.HasPositiveWCoord() ||
+      !projectedTarget.HasPositiveWCoord()) {
+    return Nothing();
+  }
+  return Some(projectedTarget.As2DPoint() - projectedAnchor.As2DPoint());
+}
+
+}  // namespace mozilla
+
+#endif