Fix TextInfo.ToTitleCase Dutch IJ digraph and U+2019 apostrophe handling

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

@@ -625,8 +625,11 @@ public string ToTitleCase(string str)
 
             StringBuilder result = new StringBuilder();
             string? lowercaseData = null;
-            // Store if the current culture is Dutch (special case)
-            bool isDutchCulture = CultureName.StartsWith("nl-", StringComparison.OrdinalIgnoreCase);
+            // Store if the current culture is Dutch (special case). This covers both the
+            // neutral culture ("nl") and any specific Dutch culture ("nl-NL", "nl-BE", etc.).
+            string cultureName = CultureName;
+            bool isDutchCulture = cultureName.StartsWith("nl", StringComparison.OrdinalIgnoreCase) &&
+                (cultureName.Length == 2 || cultureName[2] == '-');
 
             for (int i = 0; i < str.Length; i++)
             {
@@ -666,7 +669,7 @@ public string ToTitleCase(string str)
                             }
                             i += charLen;
                         }
-                        else if (str[i] == '\'')
+                        else if (IsApostrophe(str[i]))
                         {
                             i++;
                             if (hasLowerCase)
@@ -852,6 +855,18 @@ private static bool IsWordSeparator(UnicodeCategory category)
             return (c_wordSeparatorMask & (1 << (int)category)) != 0;
         }
 
+        // Characters treated as an apostrophe within a word (e.g. contractions such as
+        // "can't" or possessives such as "Grandma's"), so a following letter is not treated
+        // as the start of a new word during titlecasing:
+        //   U+0027 APOSTROPHE
+        //   U+2019 RIGHT SINGLE QUOTATION MARK (the typographic curly apostrophe)
+        //   U+2018 LEFT SINGLE QUOTATION MARK
+        //   U+FF07 FULLWIDTH APOSTROPHE
+        private static bool IsApostrophe(char c)
+        {
+            return c is '\'' or '\u2019' or '\u2018' or '\uFF07';
+        }
+
         private static bool IsLetterCategory(UnicodeCategory uc)
         {
             return uc == UnicodeCategory.UppercaseLetter

src/libraries/System.Runtime/tests/System.Globalization.Tests/System/Globalization/TextInfoTests.cs

@@ -61,9 +61,29 @@ public void ToTitleCaseTest()
             AssertExtensions.Throws<ArgumentNullException>("str", () => ti.ToTitleCase(null));
         }
 
+        [Theory]
+        // ASCII apostrophe (U+0027) keeps a contraction/possessive as a single word.
+        [InlineData("Grandma's pictures", "Grandma's Pictures")]
+        [InlineData("can't stop", "Can't Stop")]
+        // U+2019 RIGHT SINGLE QUOTATION MARK (typographic curly apostrophe).
+        [InlineData("Grandma\u2019s pictures", "Grandma\u2019s Pictures")]
+        [InlineData("can\u2019t stop", "Can\u2019t Stop")]
+        // U+2018 LEFT SINGLE QUOTATION MARK and U+FF07 FULLWIDTH APOSTROPHE.
+        [InlineData("Grandma\u2018s pictures", "Grandma\u2018s Pictures")]
+        [InlineData("Grandma\uFF07s pictures", "Grandma\uFF07s Pictures")]
+        // A genuine separator still ends the word and titlecases what follows.
+        [InlineData("Grandma-s pictures", "Grandma-S Pictures")]
+        public void ToTitleCase_Apostrophe(string input, string expected)
+        {
+            TextInfo ti = CultureInfo.GetCultureInfo("en-US").TextInfo;
+            Assert.Equal(expected, ti.ToTitleCase(input));
+        }
+
         public static IEnumerable<object[]> DutchTitleCaseInfo_TestData()
         {
             yield return new object[] { "nl-NL", "IJ IJ IJ IJ", "ij iJ Ij IJ" };
+            yield return new object[] { "nl", "IJ IJ IJ IJ", "ij iJ Ij IJ" };
+            yield return new object[] { "nl", "De IJsvogel", "de ijsvogel" };
             yield return new object[] { "nl-be", "IJzeren Eigenschappen", "ijzeren eigenschappen" };
             yield return new object[] { "NL-NL", "Lake IJssel", "lake iJssel" };
             yield return new object[] { "NL-BE", "Boba N' IJango Fett PEW PEW", "Boba n' Ijango fett PEW PEW" };