avm2: Implement TextField.getLineIndexOfChar()

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

@@ -170,10 +170,7 @@ package flash.text {
             return 0;
         }
 
-        public function getLineIndexOfChar(charIndex:int):int {
+        public native function getLineIndexOfChar(charIndex:int):int;
-            stub_method("flash.text.TextField", "getLineIndexOfChar");
-            return 0;
-        }
 
         public function getParagraphLength(charIndex:int):int {
             stub_method("flash.text.TextField", "getParagraphLength");

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

@@ -1444,3 +1444,28 @@ pub fn get_selected_text<'gc>(
     }
     Ok("".into())
 }
+
+pub fn get_line_index_of_char<'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 index = args.get_i32(activation, 0)?;
+    if index < 0 {
+        // Docs say "throw RangeError", reality says "return -1".
+        return Ok(Value::Number(-1f64));
+    }
+
+    if let Some(line) = this.line_index_of_char(index as usize) {
+        Ok(line.into())
+    } else {
+        Ok(Value::Number(-1f64))
+    }
+}

core/src/display_object/edit_text.rs

@@ -1931,6 +1931,10 @@ impl<'gc> EditText<'gc> {
         Some(first_box.start())
     }
 
+    pub fn line_index_of_char(self, index: usize) -> Option<usize> {
+        self.0.read().layout.find_line_index_by_position(index)
+    }
+
     fn execute_avm1_asfunction(
         self,
         context: &mut UpdateContext<'_, 'gc>,

core/src/html/layout.rs

@@ -10,7 +10,7 @@ use crate::tag_utils::SwfMovie;
 use crate::DefaultFont;
 use gc_arena::Collect;
 use ruffle_render::shape_utils::DrawCommand;
-use std::cmp::{max, min};
+use std::cmp::{max, min, Ordering};
 use std::fmt::{Debug, Formatter};
 use std::mem;
 use std::ops::{Deref, Range};
@@ -773,6 +773,19 @@ impl<'gc> Layout<'gc> {
             boxes_iter: None,
         }
     }
+
+    pub fn find_line_index_by_position(&self, position: usize) -> Option<usize> {
+        let result = self.lines.binary_search_by(|probe| {
+            if probe.end <= position {
+                Ordering::Less
+            } else if position < probe.start {
+                Ordering::Greater
+            } else {
+                Ordering::Equal
+            }
+        });
+        result.ok()
+    }
 }
 
 /// A `LayoutLine` represents a single line of text.