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diff --git a/layout/generic/ReflowOutput.h b/layout/generic/ReflowOutput.h
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--- /dev/null
+++ b/layout/generic/ReflowOutput.h
@@ -0,0 +1,256 @@
+/* -*- 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/. */
+
+/* struct containing the output from nsIFrame::Reflow */
+
+#ifndef mozilla_ReflowOutput_h
+#define mozilla_ReflowOutput_h
+
+#include "mozilla/EnumeratedRange.h"
+#include "mozilla/WritingModes.h"
+#include "nsBoundingMetrics.h"
+#include "nsRect.h"
+
+//----------------------------------------------------------------------
+
+namespace mozilla {
+struct ReflowInput;
+
+enum class OverflowType : uint8_t { Ink, Scrollable };
+constexpr auto AllOverflowTypes() {
+  return mozilla::MakeInclusiveEnumeratedRange(OverflowType::Ink,
+                                               OverflowType::Scrollable);
+}
+
+struct OverflowAreas {
+ public:
+  nsRect& InkOverflow() { return mInk; }
+  const nsRect& InkOverflow() const { return mInk; }
+
+  nsRect& ScrollableOverflow() { return mScrollable; }
+  const nsRect& ScrollableOverflow() const { return mScrollable; }
+
+  nsRect& Overflow(OverflowType aType) {
+    return aType == OverflowType::Ink ? InkOverflow() : ScrollableOverflow();
+  }
+  const nsRect& Overflow(OverflowType aType) const {
+    return aType == OverflowType::Ink ? InkOverflow() : ScrollableOverflow();
+  }
+
+  OverflowAreas() = default;
+
+  OverflowAreas(const nsRect& aInkOverflow, const nsRect& aScrollableOverflow)
+      : mInk(aInkOverflow), mScrollable(aScrollableOverflow) {}
+
+  bool operator==(const OverflowAreas& aOther) const {
+    // Scrollable overflow is a point-set rectangle and ink overflow
+    // is a pixel-set rectangle.
+    return InkOverflow().IsEqualInterior(aOther.InkOverflow()) &&
+           ScrollableOverflow().IsEqualEdges(aOther.ScrollableOverflow());
+  }
+
+  bool operator!=(const OverflowAreas& aOther) const {
+    return !(*this == aOther);
+  }
+
+  OverflowAreas operator+(const nsPoint& aPoint) const {
+    OverflowAreas result(*this);
+    result += aPoint;
+    return result;
+  }
+
+  OverflowAreas& operator+=(const nsPoint& aPoint) {
+    mInk += aPoint;
+    mScrollable += aPoint;
+    return *this;
+  }
+
+  void Clear() { SetAllTo(nsRect()); }
+
+  // Mutates |this| by unioning both overflow areas with |aOther|.
+  void UnionWith(const OverflowAreas& aOther);
+
+  // Mutates |this| by unioning both overflow areas with |aRect|.
+  void UnionAllWith(const nsRect& aRect);
+
+  // Mutates |this| by setting both overflow areas to |aRect|.
+  void SetAllTo(const nsRect& aRect);
+
+ private:
+  nsRect mInk;
+  nsRect mScrollable;
+};
+
+}  // namespace mozilla
+
+/**
+ * An nsCollapsingMargin represents a vertical collapsing margin between
+ * blocks as described in section 8.3.1 of CSS2,
+ * <URL: http://www.w3.org/TR/REC-CSS2/box.html#collapsing-margins >.
+ *
+ * All adjacent vertical margins collapse, and the resulting margin is
+ * the sum of the largest positive margin included and the smallest (most
+ * negative) negative margin included.
+ */
+struct nsCollapsingMargin {
+ private:
+  nscoord mMostPos;  // the largest positive margin included
+  nscoord mMostNeg;  // the smallest negative margin included
+
+ public:
+  nsCollapsingMargin() : mMostPos(0), mMostNeg(0) {}
+
+  nsCollapsingMargin(const nsCollapsingMargin& aOther) = default;
+
+  bool operator==(const nsCollapsingMargin& aOther) {
+    return mMostPos == aOther.mMostPos && mMostNeg == aOther.mMostNeg;
+  }
+
+  bool operator!=(const nsCollapsingMargin& aOther) {
+    return !(*this == aOther);
+  }
+
+  nsCollapsingMargin& operator=(const nsCollapsingMargin& aOther) = default;
+
+  void Include(nscoord aCoord) {
+    if (aCoord > mMostPos)
+      mMostPos = aCoord;
+    else if (aCoord < mMostNeg)
+      mMostNeg = aCoord;
+  }
+
+  void Include(const nsCollapsingMargin& aOther) {
+    if (aOther.mMostPos > mMostPos) mMostPos = aOther.mMostPos;
+    if (aOther.mMostNeg < mMostNeg) mMostNeg = aOther.mMostNeg;
+  }
+
+  void Zero() {
+    mMostPos = 0;
+    mMostNeg = 0;
+  }
+
+  bool IsZero() const { return (mMostPos == 0) && (mMostNeg == 0); }
+
+  nscoord get() const { return mMostPos + mMostNeg; }
+};
+
+namespace mozilla {
+
+/**
+ * ReflowOutput is initialized by a parent frame as a parameter passing to
+ * Reflow() to allow a child frame to return its desired size and alignment
+ * information.
+ *
+ * ReflowOutput's constructor usually takes a parent frame's WritingMode (or
+ * ReflowInput) because it is more convenient for the parent frame to use the
+ * stored Size() after reflowing the child frame. However, it can actually
+ * accept any WritingMode (or ReflowInput) because SetSize() knows how to
+ * convert a size in any writing mode to the stored writing mode.
+ *
+ * @see nsIFrame::Reflow() for more information.
+ */
+class ReflowOutput {
+ public:
+  explicit ReflowOutput(mozilla::WritingMode aWritingMode)
+      : mSize(aWritingMode), mWritingMode(aWritingMode) {}
+
+  // A convenient constructor to get WritingMode in ReflowInput.
+  explicit ReflowOutput(const ReflowInput& aReflowInput);
+
+  nscoord ISize(mozilla::WritingMode aWritingMode) const {
+    return mSize.ISize(aWritingMode);
+  }
+  nscoord BSize(mozilla::WritingMode aWritingMode) const {
+    return mSize.BSize(aWritingMode);
+  }
+  mozilla::LogicalSize Size(mozilla::WritingMode aWritingMode) const {
+    return mSize.ConvertTo(aWritingMode, mWritingMode);
+  }
+
+  nscoord& ISize(mozilla::WritingMode aWritingMode) {
+    return mSize.ISize(aWritingMode);
+  }
+  nscoord& BSize(mozilla::WritingMode aWritingMode) {
+    return mSize.BSize(aWritingMode);
+  }
+
+  // Set inline and block size from a LogicalSize, converting to our
+  // writing mode as necessary.
+  void SetSize(mozilla::WritingMode aWM, mozilla::LogicalSize aSize) {
+    mSize = aSize.ConvertTo(mWritingMode, aWM);
+  }
+
+  // Set both inline and block size to zero -- no need for a writing mode!
+  void ClearSize() { mSize.SizeTo(mWritingMode, 0, 0); }
+
+  // Width and Height are physical dimensions, independent of writing mode.
+  // Accessing these is slightly more expensive than accessing the logical
+  // dimensions; as far as possible, client code should work purely with logical
+  // dimensions.
+  nscoord Width() const { return mSize.Width(mWritingMode); }
+  nscoord Height() const { return mSize.Height(mWritingMode); }
+  nscoord& Width() {
+    return mWritingMode.IsVertical() ? mSize.BSize(mWritingMode)
+                                     : mSize.ISize(mWritingMode);
+  }
+  nscoord& Height() {
+    return mWritingMode.IsVertical() ? mSize.ISize(mWritingMode)
+                                     : mSize.BSize(mWritingMode);
+  }
+
+  nsSize PhysicalSize() const { return mSize.GetPhysicalSize(mWritingMode); }
+
+  // It's only meaningful to consider "ascent" on the block-start side of the
+  // frame, so no need to pass a writing mode argument
+  enum { ASK_FOR_BASELINE = nscoord_MAX };
+  nscoord BlockStartAscent() const { return mBlockStartAscent; }
+  void SetBlockStartAscent(nscoord aAscent) { mBlockStartAscent = aAscent; }
+
+  // Metrics that _exactly_ enclose the text to allow precise MathML placements.
+  nsBoundingMetrics mBoundingMetrics;  // [OUT]
+
+  // Carried out block-end margin values. This is the collapsed
+  // (generational) block-end margin value.
+  nsCollapsingMargin mCarriedOutBEndMargin;
+
+  // For frames that have content that overflow their content area
+  // (HasOverflowAreas() is true) these rectangles represent the total
+  // area of the frame including visible overflow, i.e., don't include
+  // overflowing content that is hidden.  The rects are in the local
+  // coordinate space of the frame, and should be at least as big as the
+  // desired size. If there is no content that overflows, then the
+  // overflow area is identical to the desired size and should be {0, 0,
+  // width, height}.
+  OverflowAreas mOverflowAreas;
+
+  nsRect& InkOverflow() { return mOverflowAreas.InkOverflow(); }
+  const nsRect& InkOverflow() const { return mOverflowAreas.InkOverflow(); }
+  nsRect& ScrollableOverflow() { return mOverflowAreas.ScrollableOverflow(); }
+  const nsRect& ScrollableOverflow() const {
+    return mOverflowAreas.ScrollableOverflow();
+  }
+
+  // Set all of mOverflowAreas to (0, 0, width, height).
+  void SetOverflowAreasToDesiredBounds();
+
+  // Union all of mOverflowAreas with (0, 0, width, height).
+  void UnionOverflowAreasWithDesiredBounds();
+
+  mozilla::WritingMode GetWritingMode() const { return mWritingMode; }
+
+ private:
+  // Desired size of a frame's border-box.
+  LogicalSize mSize;
+
+  // Baseline (in block direction), or the default value ASK_FOR_BASELINE.
+  nscoord mBlockStartAscent = ASK_FOR_BASELINE;
+
+  mozilla::WritingMode mWritingMode;
+};
+
+}  // namespace mozilla
+
+#endif  // mozilla_ReflowOutput_h