Fix string case-insensitive comparisons in Globalization invariant mo…

…de (#107311)

Co-authored-by: Tarek Mahmoud Sayed <tarekms@microsoft.com>

src/libraries/System.Private.CoreLib/src/System/Globalization/InvariantModeCasing.cs

@@ -215,7 +215,7 @@ internal static int CompareStringIgnoreCase(ref char strA, int lengthA, ref char
                     continue;
                 }
 
-                return (int)codePointA - (int)codePointB;
+                return (int)aUpper - (int)bUpper;
             }
 
             return lengthA - lengthB;

src/libraries/System.Runtime/tests/System.Globalization.Tests/Invariant/InvariantMode.cs

@@ -1257,5 +1257,18 @@ private static byte[] GetExpectedInvariantOrdinalSortKey(ReadOnlySpan<char> inpu
 
             return memoryStream.ToArray();
         }
+
+        [Fact]
+        public void TestChainStringComparisons()
+        {
+            var s1 = "бал";
+            var s2 = "Бан";
+            var s3 = "Д";
+
+            // If s1 < s2 and s2 < s3, then s1 < s3
+            Assert.True(string.Compare(s1, s2, StringComparison.OrdinalIgnoreCase) < 0);
+            Assert.True(string.Compare(s2, s3, StringComparison.OrdinalIgnoreCase) < 0);
+            Assert.True(string.Compare(s1, s3, StringComparison.OrdinalIgnoreCase) < 0);
+        }
     }
 }