Fix IsInRange and added unit tests. (#15079)

src/Avalonia.Base/Media/UnicodeRange.cs

@@ -131,7 +131,7 @@ namespace Avalonia.Media
         /// </returns>
         public bool IsInRange(int value)
         {
-            return value - Start <= End - Start;
+            return Start <= value && value <= End;
         }
 
         /// <summary>

tests/Avalonia.Base.UnitTests/Media/UnicodeRangeSegmentTests.cs

@@ -18,5 +18,28 @@ namespace Avalonia.Visuals.UnitTests.Media
 
             Assert.Equal(expectedEnd, segment.End);
         }
+
+        [InlineData(0)]
+        [InlineData(19)]
+        [InlineData(26)]
+        [InlineData(100)]
+        [Theory]
+        public void InRange_Should_Return_False_For_Values_Outside_Range(int value)
+        {
+            var segment = new UnicodeRangeSegment(20, 25);
+
+            Assert.Equal(false, segment.IsInRange(value));
+        }
+
+        [InlineData(20)]
+        [InlineData(21)]
+        [InlineData(22)]
+        [Theory]
+        public void InRange_Should_Return_True_For_Values_Within_Range(int value)
+        {
+            var segment = new UnicodeRangeSegment(20, 22);
+
+            Assert.Equal(true, segment.IsInRange(value));
+        }
     }
 }