avm2: Implement TextField.getLineOffset()

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

@@ -148,6 +148,8 @@ package flash.text {
 
         public native function getLineText(lineIndex:int):String;
 
+        public native function getLineOffset(lineIndex:int):int;
+
         public function getCharBoundaries(charIndex:int):Rectangle {
             stub_method("flash.text.TextField", "getCharBoundaries");
             return new Rectangle(0, 0, 1, 1);
@@ -173,11 +175,6 @@ package flash.text {
             return 0;
         }
 
-        public function getLineOffset(lineIndex:int):int {
-            stub_method("flash.text.TextField", "getLineOffset");
-            return 0;
-        }
-
         public function getParagraphLength(charIndex:int):int {
             stub_method("flash.text.TextField", "getParagraphLength");
             return 0;

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

@@ -1191,6 +1191,30 @@ pub fn get_line_text<'gc>(
     Ok(Value::Undefined)
 }
 
+pub fn get_line_offset<'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 line_num = args.get_i32(activation, 0)?;
+    if line_num < 0 {
+        return Err(make_error_2006(activation));
+    }
+
+    return if let Some(offset) = this.line_offset(line_num as usize) {
+        Ok(offset.into())
+    } else {
+        Err(make_error_2006(activation))
+    };
+}
+
 pub fn get_bottom_scroll_v<'gc>(
     _activation: &mut Activation<'_, 'gc>,
     this: Object<'gc>,

core/src/display_object/edit_text.rs

@@ -1934,6 +1934,13 @@ impl<'gc> EditText<'gc> {
         Some(line_text)
     }
 
+    pub fn line_offset(self, line: usize) -> Option<usize> {
+        let read = self.0.read();
+        let line = read.layout.lines().get(line)?;
+        let first_box = line.boxes_iter().next()?;
+        Some(first_box.start())
+    }
+
     fn execute_avm1_asfunction(
         self,
         context: &mut UpdateContext<'_, 'gc>,