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diff --git a/gfx/src/nsFontMetrics.h b/gfx/src/nsFontMetrics.h
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+++ b/gfx/src/nsFontMetrics.h
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+/* -*- Mode: C++; tab-width: 20; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* 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 NSFONTMETRICS__H__
+#define NSFONTMETRICS__H__
+
+#include <stdint.h>              // for uint32_t
+#include <sys/types.h>           // for int32_t
+#include "mozilla/Assertions.h"  // for MOZ_ASSERT_HELPER2
+#include "mozilla/RefPtr.h"      // for RefPtr
+#include "nsCOMPtr.h"            // for nsCOMPtr
+#include "nsCoord.h"             // for nscoord
+#include "nsError.h"             // for nsresult
+#include "nsFont.h"              // for nsFont
+#include "nsISupports.h"         // for NS_INLINE_DECL_REFCOUNTING
+#include "nsStyleConsts.h"
+#include "nscore.h"  // for char16_t
+
+class gfxContext;
+class gfxFontGroup;
+class gfxUserFontSet;
+class gfxTextPerfMetrics;
+class nsDeviceContext;
+class nsAtom;
+struct nsBoundingMetrics;
+struct FontMatchingStats;
+
+namespace mozilla {
+namespace gfx {
+class DrawTarget;
+}  // namespace gfx
+}  // namespace mozilla
+
+/**
+ * Font metrics
+ *
+ * This class may be somewhat misnamed. A better name might be
+ * nsFontList. The style system uses the nsFont struct for various
+ * font properties, one of which is font-family, which can contain a
+ * *list* of font names. The nsFont struct is "realized" by asking the
+ * device context to cough up an nsFontMetrics object, which contains
+ * a list of real font handles, one for each font mentioned in
+ * font-family (and for each fallback when we fall off the end of that
+ * list).
+ *
+ * The style system needs to have access to certain metrics, such as
+ * the em height (for the CSS "em" unit), and we use the first Western
+ * font's metrics for that purpose. The platform-specific
+ * implementations are expected to select non-Western fonts that "fit"
+ * reasonably well with the Western font that is loaded at Init time.
+ */
+class nsFontMetrics final {
+ public:
+  typedef mozilla::gfx::DrawTarget DrawTarget;
+
+  enum FontOrientation { eHorizontal, eVertical };
+
+  struct MOZ_STACK_CLASS Params {
+    nsAtom* language = nullptr;
+    bool explicitLanguage = false;
+    FontOrientation orientation = eHorizontal;
+    gfxUserFontSet* userFontSet = nullptr;
+    gfxTextPerfMetrics* textPerf = nullptr;
+    FontMatchingStats* fontStats = nullptr;
+    gfxFontFeatureValueSet* featureValueLookup = nullptr;
+  };
+
+  nsFontMetrics(const nsFont& aFont, const Params& aParams,
+                nsDeviceContext* aContext);
+
+  // Used by stylo
+  NS_INLINE_DECL_THREADSAFE_REFCOUNTING(nsFontMetrics)
+
+  /**
+   * Destroy this font metrics. This breaks the association between
+   * the font metrics and the device context.
+   */
+  void Destroy();
+
+  /**
+   * Return the font's x-height.
+   */
+  nscoord XHeight();
+
+  /**
+   * Return the font's cap-height.
+   */
+  nscoord CapHeight();
+
+  /**
+   * Return the font's superscript offset (the distance from the
+   * baseline to where a superscript's baseline should be placed).
+   * The value returned will be positive.
+   */
+  nscoord SuperscriptOffset();
+
+  /**
+   * Return the font's subscript offset (the distance from the
+   * baseline to where a subscript's baseline should be placed).
+   * The value returned will be positive.
+   */
+  nscoord SubscriptOffset();
+
+  /**
+   * Return the font's strikeout offset (the distance from the
+   * baseline to where a strikeout should be placed) and size.
+   * Positive values are above the baseline, negative below.
+   */
+  void GetStrikeout(nscoord& aOffset, nscoord& aSize);
+
+  /**
+   * Return the font's underline offset (the distance from the
+   * baseline to where a underline should be placed) and size.
+   * Positive values are above the baseline, negative below.
+   */
+  void GetUnderline(nscoord& aOffset, nscoord& aSize);
+
+  /**
+   * Returns the amount of internal leading for the font.
+   * This is normally the difference between the max ascent
+   * and the em ascent.
+   */
+  nscoord InternalLeading();
+
+  /**
+   * Returns the amount of external leading for the font.
+   * em ascent(?) plus external leading is the font designer's
+   * recommended line-height for this font.
+   */
+  nscoord ExternalLeading();
+
+  /**
+   * Returns the height of the em square.
+   * This is em ascent plus em descent.
+   */
+  nscoord EmHeight();
+
+  /**
+   * Returns the ascent part of the em square.
+   */
+  nscoord EmAscent();
+
+  /**
+   * Returns the descent part of the em square.
+   */
+  nscoord EmDescent();
+
+  /**
+   * Returns the height of the bounding box.
+   * This is max ascent plus max descent.
+   */
+  nscoord MaxHeight();
+
+  /**
+   * Returns the maximum distance characters in this font extend
+   * above the base line.
+   */
+  nscoord MaxAscent();
+
+  /**
+   * Returns the maximum distance characters in this font extend
+   * below the base line.
+   */
+  nscoord MaxDescent();
+
+  /**
+   * Returns the maximum character advance for the font.
+   */
+  nscoord MaxAdvance();
+
+  /**
+   * Returns the average character width
+   */
+  nscoord AveCharWidth();
+
+  /**
+   * Returns the often needed width of the space character
+   */
+  nscoord SpaceWidth();
+
+  /**
+   * Returns the font associated with these metrics. The return value
+   * is only defined after Init() has been called.
+   */
+  const nsFont& Font() const { return mFont; }
+
+  /**
+   * Returns the language associated with these metrics
+   */
+  nsAtom* Language() const { return mLanguage; }
+
+  /**
+   * Returns the orientation (horizontal/vertical) of these metrics.
+   */
+  FontOrientation Orientation() const { return mOrientation; }
+
+  int32_t GetMaxStringLength();
+
+  // Get the width for this string.  aWidth will be updated with the
+  // width in points, not twips.  Callers must convert it if they
+  // want it in another format.
+  nscoord GetWidth(const char* aString, uint32_t aLength,
+                   DrawTarget* aDrawTarget);
+  nscoord GetWidth(const char16_t* aString, uint32_t aLength,
+                   DrawTarget* aDrawTarget);
+
+  // Draw a string using this font handle on the surface passed in.
+  void DrawString(const char* aString, uint32_t aLength, nscoord aX, nscoord aY,
+                  gfxContext* aContext);
+  void DrawString(const char16_t* aString, uint32_t aLength, nscoord aX,
+                  nscoord aY, gfxContext* aContext,
+                  DrawTarget* aTextRunConstructionDrawTarget);
+
+  nsBoundingMetrics GetBoundingMetrics(const char16_t* aString,
+                                       uint32_t aLength,
+                                       DrawTarget* aDrawTarget);
+
+  // Returns the LOOSE_INK_EXTENTS bounds of the text for determing the
+  // overflow area of the string.
+  nsBoundingMetrics GetInkBoundsForInkOverflow(const char16_t* aString,
+                                               uint32_t aLength,
+                                               DrawTarget* aDrawTarget);
+
+  void SetTextRunRTL(bool aIsRTL) { mTextRunRTL = aIsRTL; }
+  bool GetTextRunRTL() const { return mTextRunRTL; }
+
+  void SetVertical(bool aVertical) { mVertical = aVertical; }
+  bool GetVertical() const { return mVertical; }
+
+  void SetTextOrientation(mozilla::StyleTextOrientation aTextOrientation) {
+    mTextOrientation = aTextOrientation;
+  }
+  mozilla::StyleTextOrientation GetTextOrientation() const {
+    return mTextOrientation;
+  }
+
+  bool ExplicitLanguage() const { return mExplicitLanguage; }
+
+  gfxFontGroup* GetThebesFontGroup() const { return mFontGroup; }
+  gfxUserFontSet* GetUserFontSet() const;
+
+  int32_t AppUnitsPerDevPixel() const { return mP2A; }
+
+ private:
+  // Private destructor, to discourage deletion outside of Release():
+  ~nsFontMetrics();
+
+  nsFont mFont;
+  RefPtr<gfxFontGroup> mFontGroup;
+  RefPtr<nsAtom> mLanguage;
+  // Pointer to the device context for which this fontMetrics object was
+  // created.
+  nsDeviceContext* MOZ_NON_OWNING_REF mDeviceContext;
+  int32_t mP2A;
+
+  // The font orientation (horizontal or vertical) for which these metrics
+  // have been initialized. This determines which line metrics (ascent and
+  // descent) they will return.
+  FontOrientation mOrientation;
+
+  // Whether mLanguage comes from explicit markup (in which case it should be
+  // used to tailor effects like case-conversion) or is an inferred/default
+  // value.
+  bool mExplicitLanguage;
+
+  // These fields may be set by clients to control the behavior of methods
+  // like GetWidth and DrawString according to the writing mode, direction
+  // and text-orientation desired.
+  bool mTextRunRTL;
+  bool mVertical;
+  mozilla::StyleTextOrientation mTextOrientation;
+};
+
+#endif /* NSFONTMETRICS__H__ */