macOS: Fix checking for undocumented cursors (#4033)

`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.

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

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

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 {