avm2: Implement TextField.getFirstCharInParagraph

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

@@ -153,10 +153,7 @@ package flash.text {
             return new Rectangle(0, 0, 1, 1);
         }
 
-        public function getFirstCharInParagraph(charIndex:int):int {
+        public native function getFirstCharInParagraph(charIndex:int):int;
-            stub_method("flash.text.TextField", "getFirstCharInParagraph");
-            return 0;
-        }
 
         public function getImageReference(id:String):DisplayObject {
             stub_method("flash.text.TextField", "getImageReference");

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

@@ -1602,3 +1602,27 @@ pub fn get_line_index_at_point<'gc>(
         Ok(Value::Number(-1f64))
     }
 }
+
+pub fn get_first_char_in_paragraph<'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_start_index_at(char_index as usize)
+        .map(|i| i as i32)
+        .unwrap_or(-1)
+        .into())
+}

core/src/display_object/edit_text.rs

@@ -1950,6 +1950,21 @@ impl<'gc> EditText<'gc> {
         self.0.read().layout.find_line_index_by_position(index)
     }
 
+    pub fn paragraph_start_index_at(self, mut index: usize) -> Option<usize> {
+        let text = self.text();
+
+        // Note that the index may equal the text length
+        if index > text.len() {
+            return None;
+        }
+
+        while index > 0 && !string_utils::swf_is_newline(text.at(index - 1)) {
+            index -= 1;
+        }
+
+        Some(index)
+    }
+
     fn execute_avm1_asfunction(
         self,
         context: &mut UpdateContext<'_, 'gc>,