Adjust behavior to match what WPF expects

src/Avalonia.Base/Media/TextFormatting/TextLineImpl.cs

@@ -204,7 +204,7 @@ namespace Avalonia.Media.TextFormatting
 
                 if (_paragraphProperties.FlowDirection == FlowDirection.RightToLeft)
                 {
-                    currentPosition = Length - firstRun.Length;
+                    currentPosition += lineLength - firstRun.Length;
                 }
 
                 return GetRunCharacterHit(firstRun, currentPosition, 0);
@@ -216,7 +216,7 @@ namespace Avalonia.Media.TextFormatting
 
                 if (_paragraphProperties.FlowDirection == FlowDirection.LeftToRight)
                 {
-                    currentPosition = FirstTextSourceIndex + Length - lastRun.Length;
+                    currentPosition += lineLength - lastRun.Length;
                 }
 
                 return GetRunCharacterHit(lastRun, currentPosition, distance);

tests/Avalonia.Skia.UnitTests/Media/TextFormatting/TextLineTests.cs

@@ -713,7 +713,7 @@ namespace Avalonia.Skia.UnitTests.Media.TextFormatting
 
                 var characterHit = textLine.GetCharacterHitFromDistance(1000);
 
-                Assert.Equal(11, characterHit.FirstCharacterIndex);
+                Assert.Equal(10, characterHit.FirstCharacterIndex);
                 Assert.Equal(1, characterHit.TrailingLength);
             }
         }