macOS: Fix selector response checking for undocumented cursors

`AnyClass::responds_to` delegates to `class_respondsToSelector`, a
function provided by the Objective-C runtime. However, at some point,
this began to return `false` for selectors referring to undocumented
cursors, despite the cursors remaining accessible via said selectors.
That this check fails prevents the cursors from being used.

We can instead send `respondsToSelector:` to the `NSCursor` class
itself. As an instance method, this is nominally impossible; however,
Apple grants an exemption[1] that permits class objects to perform
instance methods defined in the root class.

[1]: https://developer.apple.com/library/archive/documentation/Cocoa/Conceptual/ObjectiveC/Chapters/ocObjectsClasses.html#//apple_ref/doc/uid/TP30001163-CH11-TPXREF120

Checking for the undocumented cursors in this way gets them working
again, at least on macOS Sequoia 15.1.1.

src/changelog/unreleased.md

@@ -216,3 +216,4 @@ changelog entry.
 - On macOS, fixed redundant `SurfaceResized` event at window creation.
 - On macOS, fix crash when pressing Caps Lock in certain configurations.
 - On iOS, fixed `MonitorHandle`'s `PartialEq` and `Hash` implementations.
+- On macOS, fixed undocumented cursors (e.g. zoom, resize, help) always appearing to be invalid and falling back to the default cursor.

src/platform_impl/apple/appkit/cursor.rs

@@ -4,7 +4,7 @@ use std::sync::OnceLock;
 
 use objc2::rc::Retained;
 use objc2::runtime::Sel;
-use objc2::{msg_send_id, sel, ClassType};
+use objc2::{msg_send, msg_send_id, sel, ClassType};
 use objc2_app_kit::{NSBitmapImageRep, NSCursor, NSDeviceRGBColorSpace, NSImage};
 use objc2_foundation::{
     ns_string, NSData, NSDictionary, NSNumber, NSObject, NSObjectProtocol, NSPoint, NSSize,
@@ -67,7 +67,7 @@ pub(crate) fn default_cursor() -> Retained<NSCursor> {
 
 unsafe fn try_cursor_from_selector(sel: Sel) -> Option<Retained<NSCursor>> {
     let cls = NSCursor::class();
-    if cls.responds_to(sel) {
+    if msg_send![cls, respondsToSelector: sel] {
         let cursor: Retained<NSCursor> = unsafe { msg_send_id![cls, performSelector: sel] };
         Some(cursor)
     } else {