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diff --git a/gfx/src/nsBoundingMetrics.h b/gfx/src/nsBoundingMetrics.h
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+/* -*- Mode: C++; tab-width: 20; indent-tabs-mode: nil; c-basic-offset: 4 -*- */
+/* 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 __nsBoundingMetrics_h
+#define __nsBoundingMetrics_h
+
+#include "nsCoord.h"
+#include <algorithm>
+
+/* Struct used for accurate measurements of a string, in order to
+ * allow precise positioning when processing MathML.  This is in its
+ * own header file because some very-widely-included headers need it
+ * but not the rest of nsFontMetrics, or vice versa.
+ */
+
+struct nsBoundingMetrics {
+
+    ///////////
+    // Metrics that _exactly_ enclose the text:
+
+    // The character coordinate system is the one used on X Windows:
+    // 1. The origin is located at the intersection of the baseline
+    //    with the left of the character's cell.
+    // 2. All horizontal bearings are oriented from left to right.
+    // 2. All horizontal bearings are oriented from left to right.
+    // 3. The ascent is oriented from bottom to top (being 0 at the orgin).
+    // 4. The descent is oriented from top to bottom (being 0 at the origin).
+
+    // Note that Win32/Mac/PostScript use a different convention for
+    // the descent (all vertical measurements are oriented from bottom
+    // to top on these palatforms). Make sure to flip the sign of the
+    // descent on these platforms for cross-platform compatibility.
+
+    // Any of the following member variables listed here can have
+    // positive or negative value.
+
+    nscoord leftBearing;
+    /* The horizontal distance from the origin of the drawing
+       operation to the left-most part of the drawn string. */
+
+    nscoord rightBearing;
+    /* The horizontal distance from the origin of the drawing
+       operation to the right-most part of the drawn string.
+       The _exact_ width of the string is therefore:
+       rightBearing - leftBearing */
+
+    nscoord ascent;
+    /* The vertical distance from the origin of the drawing
+       operation to the top-most part of the drawn string. */
+
+    nscoord descent;
+    /* The vertical distance from the origin of the drawing
+       operation to the bottom-most part of the drawn string.
+       The _exact_ height of the string is therefore:
+       ascent + descent */
+
+    nscoord width;
+    /* The horizontal distance from the origin of the drawing
+       operation to the correct origin for drawing another string
+       to follow the current one. Depending on the font, this
+       could be greater than or less than the right bearing. */
+
+    nsBoundingMetrics() : leftBearing(0), rightBearing(0),
+                          ascent(0), descent(0), width(0)
+    {}
+
+    void
+    operator += (const nsBoundingMetrics& bm) {
+        if (ascent + descent == 0 && rightBearing - leftBearing == 0) {
+            ascent = bm.ascent;
+            descent = bm.descent;
+            leftBearing = width + bm.leftBearing;
+            rightBearing = width + bm.rightBearing;
+        }
+        else {
+            if (ascent < bm.ascent) ascent = bm.ascent;
+            if (descent < bm.descent) descent = bm.descent;
+            leftBearing = std::min(leftBearing, width + bm.leftBearing);
+            rightBearing = std::max(rightBearing, width + bm.rightBearing);
+        }
+        width += bm.width;
+    }
+};
+
+#endif // __nsBoundingMetrics_h