LibWeb: Add special handling for WindowProxy in [Replaceable] setters

Setters for Window object should consider WindowProxy wrapper by:
- Verifying `this_value` is `WindowProxy` (not `HTML::Window`)
- Defining properties on the underlying Window object instead of on
  the WindowProxy itself.

Meta/Lagom/Tools/CodeGenerators/LibWeb/BindingsGenerator/IDLGenerators.cpp

@@ -3186,6 +3186,20 @@ JS_DEFINE_NATIVE_FUNCTION(@class_name@::@attribute.setter_callback@)
 }
 )~~~");
         } else if (attribute.extended_attributes.contains("Replaceable"sv)) {
+            if (interface.name.is_one_of("Window")) {
+                attribute_generator.append(R"~~~(
+JS_DEFINE_NATIVE_FUNCTION(@class_name@::@attribute.setter_callback@)
+{
+    auto this_value = vm.this_value();
+    if (!this_value.is_object() || !is<WindowProxy>(this_value.as_object()))
+        return vm.throw_completion<JS::TypeError>(JS::ErrorType::NotAnObjectOfType, "@namespaced_name@");
+    auto& window_proxy = static_cast<WindowProxy&>(this_value.as_object());
+    TRY(window_proxy.window()->internal_define_own_property("@attribute.name@", JS::PropertyDescriptor { .value = vm.argument(0), .writable = true }));
+    return JS::js_undefined();
+}
+)~~~");
+            } else {
+
                 attribute_generator.append(R"~~~(
 JS_DEFINE_NATIVE_FUNCTION(@class_name@::@attribute.setter_callback@)
 {
@@ -3196,6 +3210,7 @@ JS_DEFINE_NATIVE_FUNCTION(@class_name@::@attribute.setter_callback@)
     return JS::js_undefined();
 }
 )~~~");
+            }
         } else if (auto put_forwards_identifier = attribute.extended_attributes.get("PutForwards"sv); put_forwards_identifier.has_value()) {
             attribute_generator.set("put_forwards_identifier"sv, *put_forwards_identifier);
 

Tests/LibWeb/Text/expected/window-event-property-setter.txt

@@ -0,0 +1 @@
+hello

Tests/LibWeb/Text/input/window-event-property-setter.html

@@ -0,0 +1,7 @@
+<script src="include.js"></script>
+<script>
+    test(() => {
+        window.event = "hello";
+        println(window.event);
+    });
+</script>