From b999f925dcfb0417c534e1a21aa74626a06b466b Mon Sep 17 00:00:00 2001
From: Shannon Booth <shannon@serenityos.org>
Date: Sun, 20 Oct 2024 13:50:39 +1300
Subject: [PATCH] LibWeb: Allow splitting surrogate pairs in
 CharacterData.substringData()

---
 ...Data-substringData-break-surrogate-pair.txt |  5 +++++
 ...ata-substringData-break-surrogate-pair.html | 18 ++++++++++++++++++
 .../Libraries/LibWeb/DOM/CharacterData.cpp     |  4 ++--
 3 files changed, 25 insertions(+), 2 deletions(-)
 create mode 100644 Tests/LibWeb/Text/expected/DOM/CharacterData-substringData-break-surrogate-pair.txt
 create mode 100644 Tests/LibWeb/Text/input/DOM/CharacterData-substringData-break-surrogate-pair.html

diff --git a/Tests/LibWeb/Text/expected/DOM/CharacterData-substringData-break-surrogate-pair.txt b/Tests/LibWeb/Text/expected/DOM/CharacterData-substringData-break-surrogate-pair.txt
new file mode 100644
index 00000000000..6a8d5b1c046
--- /dev/null
+++ b/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
diff --git a/Tests/LibWeb/Text/input/DOM/CharacterData-substringData-break-surrogate-pair.html b/Tests/LibWeb/Text/input/DOM/CharacterData-substringData-break-surrogate-pair.html
new file mode 100644
index 00000000000..fb8328b1e85
--- /dev/null
+++ b/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>
diff --git a/Userland/Libraries/LibWeb/DOM/CharacterData.cpp b/Userland/Libraries/LibWeb/DOM/CharacterData.cpp
index 07018dcd7ce..44c8080ea82 100644
--- a/Userland/Libraries/LibWeb/DOM/CharacterData.cpp
+++ b/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