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diff --git a/gfx/layers/apz/util/DoubleTapToZoom.h b/gfx/layers/apz/util/DoubleTapToZoom.h
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+++ b/gfx/layers/apz/util/DoubleTapToZoom.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 mozilla_layers_DoubleTapToZoom_h
+#define mozilla_layers_DoubleTapToZoom_h
+
+#include "Units.h"
+#include "mozilla/gfx/Matrix.h"
+
+template <class T>
+class RefPtr;
+
+namespace mozilla {
+namespace dom {
+class Document;
+}
+
+namespace layers {
+
+enum class CantZoomOutBehavior : int8_t { Nothing = 0, ZoomIn };
+
+struct ZoomTarget {
+  // The preferred target rect that we'd like to zoom in on, if possible. An
+  // empty rect means the browser should zoom out.
+  CSSRect targetRect;
+
+  // If we are asked to zoom out but cannot (due to zoom constraints, etc), then
+  // zoom in some small amount to provide feedback to the user.
+  CantZoomOutBehavior cantZoomOutBehavior = CantZoomOutBehavior::Nothing;
+
+  // If zooming all the way in on |targetRect| is not possible (for example, due
+  // to a max zoom constraint), |elementBoundingRect| may be used to inform a
+  // more optimal target scroll position (for example, we may try to maximize
+  // the area of |elementBoundingRect| that's showing, while keeping
+  // |targetRect| in view and keeping the zoom as close to the desired zoom as
+  // possible).
+  Maybe<CSSRect> elementBoundingRect;
+
+  // The document relative (ie if the content inside the root scroll frame
+  // existed without that scroll frame) pointer position at the time of the
+  // double tap or location of the double tap if we can compute it. Only used if
+  // the rest of this ZoomTarget is asking to zoom out but we are already at the
+  // minimum zoom. In which case we zoom in a small amount on this point.
+  Maybe<CSSPoint> documentRelativePointerPosition;
+};
+
+struct DoubleTapToZoomMetrics {
+  // The visual viewport rect of the top-level content document.
+  CSSRect mVisualViewport;
+  // The scrollable rect of the root scroll container of the top-level content
+  // document.
+  CSSRect mRootScrollableRect;
+  // If double-tap-to-zoom happens inside an OOP iframe, this transform matrix
+  // is the matrix converting the coordinates relative to layout viewport origin
+  // of the OOP iframe to the document origin of the top level content document.
+  // If not, this is the identity matrix.
+  CSSToCSSMatrix4x4 mTransformMatrix;
+
+  bool operator==(const DoubleTapToZoomMetrics& aOther) const {
+    return mVisualViewport == aOther.mVisualViewport &&
+           mRootScrollableRect == aOther.mRootScrollableRect &&
+           mTransformMatrix == aOther.mTransformMatrix;
+  }
+  friend std::ostream& operator<<(std::ostream& aStream,
+                                  const DoubleTapToZoomMetrics& aUpdate);
+};
+
+/**
+ * For a double tap at |aPoint|, return a ZoomTarget struct with contains a rect
+ * to which the browser should zoom in response (see ZoomTarget definition for
+ * more details). An empty rect means the browser should zoom out. |aDocument|
+ * should be the in-process root content document for the content that was
+ * tapped.
+ */
+ZoomTarget CalculateRectToZoomTo(
+    const RefPtr<mozilla::dom::Document>& aInProcessRootContentDocument,
+    const CSSPoint& aPoint, const DoubleTapToZoomMetrics& aMetrics);
+
+}  // namespace layers
+}  // namespace mozilla
+
+#endif /* mozilla_layers_DoubleTapToZoom_h */