avm2: Implement TextField.getParagraphLength

core/src/avm2/globals/flash/text/TextField.as

@@ -164,10 +164,7 @@ package flash.text {
 
         public native function getLineIndexOfChar(charIndex:int):int;
 
-        public function getParagraphLength(charIndex:int):int {
+        public native function getParagraphLength(charIndex:int):int;
-            stub_method("flash.text.TextField", "getParagraphLength");
-            return 0;
-        }
 
         public static function isFontCompatible(fontName:String, fontStyle:String):Boolean {
             stub_method("flash.text.TextField", "isFontCompatible");

core/src/avm2/globals/flash/text/text_field.rs

@@ -1626,3 +1626,27 @@ pub fn get_first_char_in_paragraph<'gc>(
         .unwrap_or(-1)
         .into())
 }
+
+pub fn get_paragraph_length<'gc>(
+    activation: &mut Activation<'_, 'gc>,
+    this: Object<'gc>,
+    args: &[Value<'gc>],
+) -> Result<Value<'gc>, Error<'gc>> {
+    let Some(this) = this
+        .as_display_object()
+        .and_then(|this| this.as_edit_text())
+    else {
+        return Ok(Value::Undefined);
+    };
+
+    let char_index = args.get_i32(activation, 0)?;
+    if char_index < 0 {
+        return Ok((-1).into());
+    }
+
+    Ok(this
+        .paragraph_length_at(char_index as usize)
+        .map(|i| i as i32)
+        .unwrap_or(-1)
+        .into())
+}

core/src/display_object/edit_text.rs

@@ -1965,6 +1965,30 @@ impl<'gc> EditText<'gc> {
         Some(index)
     }
 
+    pub fn paragraph_length_at(self, mut index: usize) -> Option<usize> {
+        let start_index = self.paragraph_start_index_at(index)?;
+        let text = self.text();
+        let length = text.len();
+
+        // When the index is equal to the text length,
+        // FP simulates a character at that point and returns
+        // the length of the last paragraph plus one.
+        if index == length {
+            return Some(1 + length - start_index);
+        }
+
+        while index < length && !string_utils::swf_is_newline(text.at(index)) {
+            index += 1;
+        }
+
+        // The trailing newline also counts to the length
+        if index < length && string_utils::swf_is_newline(text.at(index)) {
+            index += 1;
+        }
+
+        Some(index - start_index)
+    }
+
     fn execute_avm1_asfunction(
         self,
         context: &mut UpdateContext<'_, 'gc>,