fix: Don't reset the view's class on macOS when KVO has subclassed it

adapters/macos/Cargo.toml

@@ -36,3 +36,7 @@ objc2-app-kit = { version = "0.2.0", features = [
     "NSView",
     "NSWindow",
 ] }
+
+[[test]]
+name = "kvo_teardown"
+harness = false

adapters/macos/src/subclass.rs

@@ -256,10 +256,24 @@ impl SubclassingAdapter {
     }
 }
 
+fn has_our_subclass(view: &NSView, prev_class: &'static AnyClass) -> bool {
+    let current: *const AnyClass = view.class();
+    let subclasses = SUBCLASSES.lock().unwrap();
+    subclasses
+        .iter()
+        .any(|entry| std::ptr::eq(entry.0, prev_class) && std::ptr::eq(entry.1, current))
+}
+
 impl Drop for SubclassingAdapter {
     fn drop(&mut self) {
         let prev_class = self.associated.ivars().prev_class;
         let view = Id::as_ptr(&self.view) as *mut NSView;
+        // KVO may have installed its own subclass on top of ours (AppKit
+        // observes the first responder). Resetting the class would break
+        // those observers, so leave the view as it is in that case.
+        if !has_our_subclass(unsafe { &*view }, prev_class) {
+            return;
+        }
         unsafe { object_setClass(view as *mut _, (prev_class as *const AnyClass).cast()) };
         unsafe {
             objc_setAssociatedObject(

adapters/macos/tests/kvo_teardown.rs

@@ -0,0 +1,85 @@
+// Copyright 2026 The AccessKit Authors. All rights reserved.
+// Licensed under the Apache License, Version 2.0 (found in
+// the LICENSE-APACHE file) or the MIT license (found in
+// the LICENSE-MIT file), at your option.
+
+// This test doesn't use the default harness, because the adapter
+// must be created on the main thread.
+
+use std::ffi::c_void;
+
+use accesskit::{ActionHandler, ActionRequest, ActivationHandler, TreeUpdate};
+use accesskit_macos::SubclassingAdapter;
+use objc2::rc::Id;
+use objc2::runtime::AnyObject;
+use objc2::{class, msg_send, msg_send_id};
+use objc2_app_kit::NSView;
+use objc2_foundation::{NSArray, NSObject, NSString};
+
+struct NoTree;
+
+impl ActivationHandler for NoTree {
+    fn request_initial_tree(&mut self) -> Option<TreeUpdate> {
+        None
+    }
+}
+
+struct NoActions;
+
+impl ActionHandler for NoActions {
+    fn do_action(&mut self, _request: ActionRequest) {}
+}
+
+fn new_view() -> Id<NSView> {
+    unsafe { msg_send_id![class!(NSView), new] }
+}
+
+fn adapter_for(view: &NSView) -> SubclassingAdapter {
+    unsafe { SubclassingAdapter::new(view as *const NSView as *mut c_void, NoTree, NoActions) }
+}
+
+fn class_name(view: &NSView) -> String {
+    let object: &AnyObject = view;
+    object.class().name().to_owned()
+}
+
+fn an_unobserved_view_gets_its_class_back() {
+    let view = new_view();
+    let adapter = adapter_for(&view);
+    assert_eq!(class_name(&view), "AccessKitSubclassOfNSView");
+    drop(adapter);
+    assert_eq!(class_name(&view), "NSView");
+    drop(adapter_for(&view));
+    assert_eq!(class_name(&view), "NSView");
+}
+
+// AppKit observes the first responder and can remove its observers
+// after the adapter is dropped.
+fn an_observed_view_keeps_its_observers() {
+    let view = new_view();
+    let adapter = adapter_for(&view);
+    let observer = NSObject::new();
+    let key_path = NSString::from_str("nextResponder");
+    let no_context: *mut c_void = std::ptr::null_mut();
+    let _: () = unsafe {
+        msg_send![&view, addObserver: &*observer, forKeyPath: &*key_path, options: 0usize, context: no_context]
+    };
+    assert_eq!(
+        class_name(&view),
+        "NSKVONotifying_AccessKitSubclassOfNSView"
+    );
+
+    drop(adapter);
+    assert_eq!(
+        class_name(&view),
+        "NSKVONotifying_AccessKitSubclassOfNSView"
+    );
+    let _children: *mut NSArray<NSObject> = unsafe { msg_send![&view, accessibilityChildren] };
+    let _: () = unsafe { msg_send![&view, removeObserver: &*observer, forKeyPath: &*key_path] };
+    assert_eq!(class_name(&view), "AccessKitSubclassOfNSView");
+}
+
+fn main() {
+    an_unobserved_view_gets_its_class_back();
+    an_observed_view_keeps_its_observers();
+}