Mac Catalyst: Get the NSWindow from a UIWindow (Updated for macOS 11 Big Sur, also works with Catalina)

UIWindow+PSPDFAdditions.h

// Don't forget to prefix your category!

#import <UIKit/UIKit.h>

NS_ASSUME_NONNULL_BEGIN

@interface UIWindow (PSPDFAdditions)

#if TARGET_OS_UIKITFORMAC

/**
    Finds the NSWindow hosting the UIWindow.
    @note This is a hack. Iterates over all windows to find match. Might fail.
 */
@property (nonatomic, readonly, nullable) id nsWindow;

#endif

@end

NS_ASSUME_NONNULL_END

UIWindow+PSPDFAdditions.m

#import "UIWindow+PSPDFAdditions.h"

@implementation UIWindow (PSPDFAdditions)

#if TARGET_OS_UIKITFORMAC

- (nullable NSObject *)nsWindow {
    id delegate = [[NSClassFromString(@"NSApplication") sharedApplication] delegate];
    const SEL hostWinSEL = NSSelectorFromString([NSString stringWithFormat:@"_%@Window%@Window:", @"host", @"ForUI"]);
    @try {
        // There's also hostWindowForUIWindow 🤷‍♂️
        PSPDF_SILENCE_CALL_TO_UNKNOWN_SELECTOR(id nsWindow = [delegate performSelector:hostWinSEL withObject:self];)
            
        // macOS 11.0 changed this to return an UINSWindowProxy
        let attachedWin = NSSelectorFromString([NSString stringWithFormat:@"%@%@", @"attached", @"Window"]);
        if ([nsWindow respondsToSelector:attachedWin]) {
            nsWindow = [nsWindow valueForKey:NSStringFromSelector(attachedWin)];
        }
        
        return nsWindow;
    } @catch (...) {
        NSLog(@"Failed to get NSWindow for %@.", self);
    }
    return nil;
}

#endif

@end

This works great. just need to be sure to let the UI settle down with delays before you try to access the window. Using these to lock the aspect ratio in a MacCatalyst app:

delay(2) {
    cover.removeFromSuperview()
    #if targetEnvironment(macCatalyst)
    let ns = window.nsWindow
    let frame = ns?.value(forKey: "frame")
    let size = (frame as! CGRect).size
    ns!.setValue(CGSize(1.0, size.height/size.width), forKey: "aspectRatio")
    #endif
}