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diff --git a/layout/base/LayoutConstants.h b/layout/base/LayoutConstants.h
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+++ b/layout/base/LayoutConstants.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/. */
+
+/* constants used throughout the Layout module */
+
+#ifndef LayoutConstants_h___
+#define LayoutConstants_h___
+
+#include "mozilla/EnumSet.h"
+#include "Units.h"
+
+/**
+ * Constant used to indicate an unconstrained size.
+ *
+ * NOTE: The constants defined in this file are semantically used as symbolic
+ *       values, so user should not depend on the underlying numeric values. If
+ *       new specific use cases arise, define a new constant here.
+ */
+inline constexpr nscoord NS_UNCONSTRAINEDSIZE = nscoord_MAX;
+
+// NS_AUTOOFFSET is assumed to have the same value as NS_UNCONSTRAINEDSIZE.
+inline constexpr nscoord NS_AUTOOFFSET = NS_UNCONSTRAINEDSIZE;
+
+// +1 is to avoid clamped huge margin values being processed as auto margins
+inline constexpr nscoord NS_AUTOMARGIN = NS_UNCONSTRAINEDSIZE + 1;
+
+inline constexpr nscoord NS_INTRINSIC_ISIZE_UNKNOWN = nscoord_MIN;
+
+namespace mozilla {
+
+/**
+ * Bit-flags to pass to various functions that compute sizes like
+ * nsIFrame::ComputeSize().
+ */
+enum class ComputeSizeFlag : uint8_t {
+  /**
+   * Set if the frame is in a context where non-replaced blocks should
+   * shrink-wrap (e.g., it's floating, absolutely positioned, or
+   * inline-block).
+   */
+  ShrinkWrap,
+
+  /**
+   * Set if this is a grid measuring reflow, to prevent stretching.
+   */
+  IsGridMeasuringReflow,
+
+  /**
+   * Indicates that we should clamp the margin-box min-size to the given CB
+   * size.  This is used for implementing the grid area clamping here:
+   * https://drafts.csswg.org/css-grid/#min-size-auto
+   */
+  IClampMarginBoxMinSize,  // clamp in our inline axis
+  BClampMarginBoxMinSize,  // clamp in our block axis
+
+  /**
+   * The frame is stretching (per CSS Box Alignment) and doesn't have an
+   * Automatic Minimum Size in the indicated axis.
+   * (may be used for both flex/grid items, but currently only used for Grid)
+   * https://drafts.csswg.org/css-grid/#min-size-auto
+   * https://drafts.csswg.org/css-align-3/#valdef-justify-self-stretch
+   */
+  IApplyAutoMinSize,  // Only has an effect when the ShrinkWrap bit is unset.
+};
+using ComputeSizeFlags = mozilla::EnumSet<ComputeSizeFlag>;
+
+/**
+ * The fallback size of width is 300px and the aspect-ratio is 2:1, based on
+ * CSS2 section 10.3.2 and CSS Sizing Level 3 section 5.1:
+ * https://drafts.csswg.org/css2/visudet.html#inline-replaced-width
+ * https://drafts.csswg.org/css-sizing-3/#intrinsic-sizes
+ */
+inline constexpr CSSIntCoord kFallbackIntrinsicWidthInPixels(300);
+inline constexpr CSSIntCoord kFallbackIntrinsicHeightInPixels(150);
+inline constexpr CSSIntSize kFallbackIntrinsicSizeInPixels(
+    kFallbackIntrinsicWidthInPixels, kFallbackIntrinsicHeightInPixels);
+
+inline constexpr nscoord kFallbackIntrinsicWidth =
+    kFallbackIntrinsicWidthInPixels * AppUnitsPerCSSPixel();
+inline constexpr nscoord kFallbackIntrinsicHeight =
+    kFallbackIntrinsicHeightInPixels * AppUnitsPerCSSPixel();
+inline constexpr nsSize kFallbackIntrinsicSize(kFallbackIntrinsicWidth,
+                                               kFallbackIntrinsicHeight);
+
+/**
+ * This is used in some nsLayoutUtils functions.
+ * Declared here so that fewer files need to include nsLayoutUtils.h.
+ */
+enum class IntrinsicISizeType { MinISize, PrefISize };
+
+enum class ContentRelevancyReason {
+  // If the content of this Frame is on screen or nearly on screen.
+  Visible,
+
+  // If this Frame's element has focus in its subtree.
+  FocusInSubtree,
+
+  // If this Frame's content is part of a selection.
+  Selected,
+};
+using ContentRelevancy = EnumSet<ContentRelevancyReason, uint8_t>;
+
+}  // namespace mozilla
+
+#endif  // LayoutConstants_h___