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diff --git a/src/Text/Def/TextUI/Editor/IScrollMap.cs b/src/Text/Def/TextUI/Editor/IScrollMap.cs
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+++ b/src/Text/Def/TextUI/Editor/IScrollMap.cs
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+// Copyright (c) Microsoft Corporation
+// All rights reserved
+
+namespace Microsoft.VisualStudio.Text.Editor
+{
+    using System;
+    using Microsoft.VisualStudio.Text;
+
+    /// <summary>
+    /// <para>Defines the mapping between character positions and scrollmap coordinates. This is not
+    /// the same as the coordinate system in which the scrollbar is rendered.</para>
+    /// </summary>
+    /// <remarks>
+    /// <para>Valid text positions range are [0...TextView.TextSnapshot.Length].</para>
+    /// <para>Corresponding scrollmap coordinates are [0.0 ... CoordinateOfBufferEnd].</para>
+    /// <para>Not every buffer position will have a distinct scrollmap coordinate. For example, every character on the same line of text will,
+    /// generally, have the same scrollmap coordinate.</para>
+    /// <para>Different scrollmap coordinates may map to the same buffer position. For example, scrollmap coordinates in the range [0.0, 1.0) will, generally,
+    /// map to the first character of the buffer.</para>
+    /// </remarks>
+    public interface IScrollMap : IVerticalFractionMap
+    {
+        /// <summary>
+        /// Gets the scrollmap coordinates of a buffer position.
+        /// </summary>
+        /// <param name="bufferPosition">The buffer position.</param>
+        /// <returns>The scrollmap coordinate, which will be between 0.0 and CoordinateOfBufferEnd inclusive.</returns>
+        double GetCoordinateAtBufferPosition(SnapshotPoint bufferPosition);
+
+        /// <summary>
+        /// Does the coordinate system used by this scroll map act as if all elisions are expanded?
+        /// </summary>
+        bool AreElisionsExpanded { get; }
+
+        /// <summary>
+        /// Gets the buffer position that corresponds to a scrollmap coordinate.
+        /// </summary>
+        /// <param name="coordinate">The scrollmap coordinate.</param>
+        /// <returns>The corresponding buffer position.</returns>
+        /// <exception cref="ArgumentOutOfRangeException"><paramref name="coordinate"/> is NaN.</exception>
+        /// <remarks>Different buffer positions can have the same scroll map coordinates. This method is guaranteed only to be consistent: it will
+        /// return the same position for the same coordinate. The exact character returned will depend on the implementation of the scroll map.
+        /// It will generally be the first character on the line.</remarks>
+        SnapshotPoint GetBufferPositionAtCoordinate(double coordinate);
+
+        /// <summary>
+        /// The scrollmap coordinate of the start of the buffer.
+        /// </summary>
+        double Start { get; }
+
+        /// <summary>
+        /// The scrollmap coordinate of the end of the buffer.
+        /// </summary>
+        double End { get; }
+
+        /// <summary>
+        /// Gets the size of the text visible in the view (in scrollmap coordinates).
+        /// </summary>
+        /// <remarks>
+        /// This is equivalent to the scrollbar thumb size. The total height of the scroll map, in scrollmap coordinates, 
+        /// is CoordinateOfBufferEnd + ThumbSize.
+        /// </remarks>
+        double ThumbSize { get; }
+    }
+}