SKCropNode issue

Question

Spent a whole day on this. I don't find this in the iOS docs or on SO.

I have a SKShapeNode* that is like a window in my app, and I add a background that is an SKSpriteNode, and the background has another 10 SKSpriteNode as its children. So the node tree is like this:

SKScene -> window -> background -> (child1, ..., child10)

The dimensions are such that the background matches the size of the window, and all the background's children fit inside the background.

I want to zoom when I click inside the window (have the background & 10 children all zoom together). I accomplish this by setting the background's xScale & yScale, and the children inherit this scaling. But I also don't want to spill outside the window's boundaries, so I made a SKCropNode, and added the background as its child. Now the background doesn't spill out:

SKScene -> window -> SKCropNode -> background -> (child1, ..., child10)

Problem is, the background's children spill out when zooming. This is counter-intuitive to me. I tried searching online and looking in docs, "does SKCropNode crop its children & all descendants"? Since the answer appears to be no, I thought to change all 10 children's parent from background to SKCropNode:

SKScene -> window -> SKCropNode -> (background, child1, ..., child10)

Now I scale the SKCropNode. This scales background and all children, but now it spills to outside the window again. (Later in the game, the number of children may increase from 10 to 300, and I don't want to do a for loop on 300 items. So I want to be able to set scale on just one parent.)

I finally decided to try something a bit "hacky". This I did not find anywhere online, so I'm wondering if I'm in "undefined behavior" territory.

SKScene -> window -> SKCropNode1 -> SKCropNode2 -> (background, child1, ..., child10)

I added another SKCropNode on top of my original SKCropNode. Now, I only scale SKCropNode2. This works. However, now I'm getting very strange behavior. My SKShapeNode buttons (completely outside the window) will disappear one by one, then come back, and cycle like this. Further, the "nodes: 10, 60.0 fps" in the lower right will disappear too and return in the cycle. By cycle I mean me clicking inside the window zooming. It seems I've hit a bug in SpriteKit? I set the zPosition of the buttons to 20, way higher than anything else (5 and below). I also set skview.ignoresSiblingOrder = false; Appreciate any help or advice on how to accomplish this!

Edit: In reply to the comments, I did not use a simulator. I tested this on my iPad Pro and iPhone 6+, both running iOS 9.2. Below is my code that compiles & reproduces the behavior. I also took out the zooming code, but it's still reproducible. Please try to tap on the spaceship (Apple's sample image) about 30 times, you will start to see it then.

MainScene.h

#import <UIKit/UIKit.h>
#import <SpriteKit/SpriteKit.h>
@interface MainScene : SKScene<NSStreamDelegate>
@property (strong, nonatomic) SKCropNode* skcrop;
@end

MainScene.m

#import "MainScene.h"

@implementation MainScene
- (void)didMoveToView: (SKView*)view { }

-(void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {
    if ((int)[touches count] != 1) return;
    UITouch* touch = [touches anyObject];

    const CGPoint location = [touch locationInNode:self];

    { // without the 6 lines below, the disappearing-sprites behavior is gone
        SKShapeNode* newshape = [SKShapeNode shapeNodeWithRectOfSize:
            CGSizeMake(10.0, 10.0) cornerRadius:1.0];
        newshape.position = location;
        newshape.zPosition = 5;
        newshape.fillColor = [UIColor purpleColor];
        [self.skcrop addChild:newshape];
    }
}

-(void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event { }

-(void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event { }

@end

GameViewController.m:

#import "GameViewController.h"
#import "MainScene.h"
#import <CoreFoundation/CoreFoundation.h>

@implementation GameViewController

- (UIInterfaceOrientationMask)supportedInterfaceOrientations {
    return UIInterfaceOrientationMaskLandscapeLeft
        | UIInterfaceOrientationMaskLandscapeRight;
}
- (void)viewDidLoad {
    [super viewDidLoad];

    // Configure SKView
    SKView* skview = (SKView*)self.view;
    skview.showsFPS = true;
    skview.showsNodeCount = true;
    skview.ignoresSiblingOrder = false;
    skview.multipleTouchEnabled = false;

    // Get Screen Size
    // IPad Pro  prints: screen size 768 1024
    // IPhone 6+ prints: screen size 375  667
    const int screenWidth = floor(0.5+skview.bounds.size.width);
    const int screenHeight = floor(0.5+skview.bounds.size.height);
    NSLog(@"screen size %d %d", screenWidth, screenHeight);
    const double width = (screenWidth < 375) ? 360 : 720;

    // Configure SKScene
    MainScene *skscene = [[MainScene alloc]
        initWithSize:CGSizeMake(screenWidth, screenHeight)];
    skscene.scaleMode = SKSceneScaleModeFill;
    skscene.backgroundColor = [UIColor whiteColor];
    [skview presentScene:skscene];

    // Set up window's crop mask
    const CGSize winSurface = CGSizeMake(width, width);
    const CGPoint winPosition = CGPointMake(
            CGRectGetMidX(skscene.frame), CGRectGetMidY(skscene.frame));
    NSLog(@"pos %f %f", winPosition.x, winPosition.y);

    SKSpriteNode* winMaskParent = [[SKSpriteNode alloc]
        initWithColor:[UIColor redColor] size:winSurface];
    [winMaskParent retain];
    winMaskParent.position = winPosition;
    SKCropNode* scnParent = [SKCropNode node];
    scnParent.zPosition = 1;
    scnParent.maskNode = winMaskParent;
    [skscene addChild:scnParent];

    SKSpriteNode* winMask = [[SKSpriteNode alloc]
        initWithColor:[UIColor blueColor] size:winSurface];
    [winMask retain];
    winMask.position = winPosition;
    SKCropNode* scn = [SKCropNode node];
    scn.zPosition = 1;
    scn.maskNode = winMask;
    [scnParent addChild:scn];

    // Add window sprite
    SKSpriteNode* win =
        [SKSpriteNode spriteNodeWithImageNamed:@"Spaceship.png"];
    win.zPosition = 2;
    win.position = winPosition;
    [scn addChild:win];

    for (int i = 0; i < 5; ++i) {
        const double height = 30.0;
        const double width = 50.0;
        const double posY = screenHeight - (1+i)*100.0;
        const double posX = screenWidth - width - 10.0;

        SKShapeNode* button = [SKShapeNode shapeNodeWithRectOfSize:
            CGSizeMake(width, height) cornerRadius:1.0];
        button.position = CGPointMake(posX, posY);
        button.zPosition = 15;
        button.fillColor = [UIColor greenColor];
        button.lineWidth = 1.0;
        button.glowWidth = 0.0;
        [skscene addChild:button];
    }

    skscene.skcrop = scn;
    return;
}

@end

Edit 2: I removed the nested SKCropNode so that there is only 1 layer of SKCropNode. The button sprites disappear after a few clicks on the spaceship.