LibWeb: Allow splitting surrogate pairs in CharacterData.substringData()

Author: https://github.com/shannonbooth
Commit: b999f925dc
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/1875
Reviewed-by: https://github.com/tcl3 ✅

Tests/LibWeb/Text/expected/DOM/CharacterData-substringData-break-surrogate-pair.txt

@@ -0,0 +1,5 @@
+Before substringData:
+[0]: 55357
+[1]: 56374
+After substringData(0, 1):
+[0]: 55357

Tests/LibWeb/Text/input/DOM/CharacterData-substringData-break-surrogate-pair.html

@@ -0,0 +1,18 @@
+<script src="../include.js"></script>
+<script>
+    test(() => {
+        const str = '\uD83D\uDC36'; // 🐶
+        let t = document.createTextNode(str);
+        println("Before substringData:");
+        for (let i = 0; i < t.length; ++i) {
+            println("[" + i + "]: " + t.data.charCodeAt(i));
+        }
+
+        // Break the surrogate pair
+        const invalidSubstring = t.substringData(0, 1);
+        println("After substringData(0, 1):");
+        for (let i = 0; i < invalidSubstring.length; ++i) {
+            println("[" + i + "]: " + invalidSubstring.charCodeAt(i));
+        }
+    });
+</script>

Userland/Libraries/LibWeb/DOM/CharacterData.cpp

@@ -57,10 +57,10 @@ WebIDL::ExceptionOr<String> CharacterData::substring_data(size_t offset, size_t
     // 3. If offset plus count is greater than length, return a string whose value is the code units from the offsetth code unit
     //    to the end of node’s data, and then return.
     if (offset + count > length)
-        return MUST(utf16_view.substring_view(offset).to_utf8());
+        return MUST(utf16_view.substring_view(offset).to_utf8(Utf16View::AllowInvalidCodeUnits::Yes));
 
     // 4. Return a string whose value is the code units from the offsetth code unit to the offset+countth code unit in node’s data.
-    return MUST(utf16_view.substring_view(offset, count).to_utf8());
+    return MUST(utf16_view.substring_view(offset, count).to_utf8(Utf16View::AllowInvalidCodeUnits::Yes));
 }
 
 // https://dom.spec.whatwg.org/#concept-cd-replace