Make GritSplitter calculations match the ones in WPF.

src/Avalonia.Base/Utilities/MathUtilities.cs

@@ -30,6 +30,21 @@ namespace Avalonia.Utilities
             return (-eps < delta) && (eps > delta);
         }
 
+        /// <summary>
+        /// AreClose - Returns whether or not two doubles are "close".  That is, whether or
+        /// not they are within epsilon of each other.
+        /// </summary>
+        /// <param name="value1"> The first double to compare. </param>
+        /// <param name="value2"> The second double to compare. </param>
+        /// <param name="eps"> The fixed epsilon value used to compare.</param>
+        public static bool AreClose(double value1, double value2, double eps)
+        {
+            //in case they are Infinities (then epsilon check does not work)
+            if (value1 == value2) return true;
+            double delta = value1 - value2;
+            return (-eps < delta) && (eps > delta);
+        }
+
         /// <summary>
         /// AreClose - Returns whether or not two floats are "close".  That is, whether or 
         /// not they are within epsilon of each other.

src/Avalonia.Controls/GridSplitter.cs

@@ -632,11 +632,11 @@ namespace Avalonia.Controls
                 double actualLength2 = GetActualLength(definition2);
 
                 // When splitting, Check to see if the total pixels spanned by the definitions 
-                // is the same asbefore starting resize. If not cancel the drag
+                // is the same as before starting resize. If not cancel the drag.
                 if (_resizeData.SplitBehavior == SplitBehavior.Split &&
                     !MathUtilities.AreClose(
                         actualLength1 + actualLength2,
-                        _resizeData.OriginalDefinition1ActualLength + _resizeData.OriginalDefinition2ActualLength))
+                        _resizeData.OriginalDefinition1ActualLength + _resizeData.OriginalDefinition2ActualLength, LayoutHelper.LayoutEpsilon))
                 {
                     CancelResize();
 

src/Avalonia.Layout/LayoutHelper.cs

@@ -9,6 +9,12 @@ namespace Avalonia.Layout
     /// </summary>
     public static class LayoutHelper
     {
+        /// <summary>
+        /// Epsilon value used for certain layout calculations.
+        /// Based on the value in WPF LayoutDoubleUtil.
+        /// </summary>
+        public const double LayoutEpsilon = 0.00000153;
+
         /// <summary>
         /// Calculates a control's size based on its <see cref="ILayoutable.Width"/>,
         /// <see cref="ILayoutable.Height"/>, <see cref="ILayoutable.MinWidth"/>,