For 100% macOS app without storyboard and mainmenu xib you also need to connect the main() function to call the AppDelegate , this way the NSApplication will call applicationDidFinishLaunching in AppDelegate. so this is the main file : `#import <Cocoa/Cocoa.h> #import "AppDelegate.h" AppDelegate * delegate; int main(int argc, const char * argv[]) {         delegate =  [[AppDelegate alloc] init];         NSApplication.sharedApplication.delegate = delegate;     return NSApplicationMain(argc, argv); }`

short answer : NSCollectionLayoutGroup customGroupWithLayoutSize then, inside the block we need to return -ALL- items frames any time the system ask for us. next code will create custom group, centered, each row is height for ((i+1)*10) // just for testing and learning     NSCollectionLayoutGroup * group = [NSCollectionLayoutGroup customGroupWithLayoutSize:groupSize itemProvider:^NSArray<NSCollectionLayoutGroupCustomItem *> * _Nonnull(id<NSCollectionLayoutEnvironment>  _Nonnull layoutEnvironment) {                          NSMutableArray<NSCollectionLayoutGroupCustomItem*>* customItems = [@[]mutableCopy];                  CGFloat x=0, y=0;         CGFloat height;         CGFloat horizontal_edge_space = ceil((layoutEnvironment.container.contentSize.width-300) / 2);                  for(int i =0; i<100 ; i++)         {             height = (i+1) * 10;             NSCollectionLayoutGroupCustomItem * customItem =[NSCollectionLayoutGroupCustomItem customItemWithFrame:CGRectMake(horizontal_edge_space,y,300,height)];             [customItems addObject:customItem];             y=y+height;         }         return [customItems copy];     }];

Hi darkmaterial, I'm not answering you question directly, cause I don't remember exactly what you are missing. but, I want to write some info I wrote for my self next time I'm going to deal with CALayers and AppKit hope it helps, read and check it, good chance the answer is there : Difference between uiview-layer and nsview layer https://stackoverflow.com/questions/9551992/how-to-add-a-calayer-to-an-nsview-on-mac-os-x https://stackoverflow.com/questions/15966219/calayer-drawrect Very important slide show of WWDC 2012 about NSView layers and drawing https://docs.huihoo.com/apple/wwdc/2012/session_217__layerbacked_views_appkit__core_animation.pdf https://www.objc.io/issues/14-mac/appkit-for-uikit-developers/ By default, AppKit views are not backed by Core Animation layers; layer-backing support has been integrated into AppKit retroactively. But while you never have to worry about this with UIKit, with AppKit there are decisions to make. AppKit differentiates between layer-backed and layer-hosting views If you want to interact with the layer in such ways, then you have to go one step further. Overriding NSView‘s wantsUpdateLayer method to return YES enables you to change the layer’s properties. If you do this though, AppKit will no longer call the view’s drawRect: method. Instead, updateLayer will be called during the view update cycle, and this is where you can modify the layer. To make appKit layers works like UIKit layers wantsLayer = YES wantsUpdateLayer return  YES And now - (void)updateLayer will be called instead of drawRect: Make sure layerContentsRedrawPolicy property to NSViewLayerContentsRedrawOnSetNeedsDisplay This way, you have control over when the layer contents need to be redrawn. A frame change will not automatically trigger a redraw anymore; you are now responsible for triggering it by calling -setNeedsDisplay: Again all the above is note I have for my self, incase I want to do something with CALayers ( rarely). I hope this help you. Good luck

Also updated to Big Sur 11.2 (Mac mini 2018) Same problem. Also tried to change to from Debug to Release. Not working.

Found the problem :-(BOOL)applicationShouldTerminateAfterLastWindowClosed:(NSApplication *)sender { return YES; }when I disable this,I could hide/unhide the my app and activate it correctly with 3 buttons of window with their colors.Another thing import was :[self.windowController.window orderOut:nil];I wish AppKit and xcode were more developer friendly.