title: NSCollectionView does not scroll items past initial visible rect
body: <p>I'm running into an issue with an existing app while trying to fix a few outstanding macOS 10.13 bugs. I have a small NSCollectionView that looks similar to the small monthly calendar in the Calendar app. It displays days and scrolls vertically on click. However, on macOS 10.13 the collection view only displays a few rows and does not scroll. I've verified that the data source is being called correctly and it does try to load additional items - but doesn't scroll to them.</p>
  
  <p>I've created a small sample application that also demonstrates the issue. This is a basic macOS app that adds an NSCollectionView via the Main storyboard and has a generic NSCollectionViewItem class loaded from a nib. The entirety of the code in the main view controller is:</p>
  
  <pre><code>- (void)viewDidLoad {  
    [super viewDidLoad];  
  
    NSNib *nib = [[NSNib alloc] initWithNibNamed:@"FooCollectionViewItem" bundle:nil];  
    [self.collectionView registerNib:nib forItemWithIdentifier:@"foo"];  
  }  
  - (void)viewDidAppear {  
    [super viewDidAppear];  
  
    [self.collectionView reloadData];  
  }  
  - (void)setRepresentedObject:(id)representedObject {  
    [super setRepresentedObject:representedObject];  
  
  }  
  - (NSInteger)numberOfSectionsInCollectionView:(NSCollectionView *)collectionView {  
    return 1;  
  }  
  - (NSInteger)collectionView:(NSCollectionView *)collectionView numberOfItemsInSection:(NSInteger)section {  
    return 2000;  
  }  
  - (NSCollectionViewItem *)collectionView:(NSCollectionView *)collectionView itemForRepresentedObjectAtIndexPath:(NSIndexPath *)indexPath {  
    FooCollectionViewItem *item = [collectionView makeItemWithIdentifier:@"foo" forIndexPath:indexPath];  
    item.fooField.stringValue = [NSString stringWithFormat:@"%ld", indexPath.item];  
  
    return item;  
  }
  </code></pre>
  
  <p>The resulting app looks like:</p>
  
  <p><a href="https://i.stack.imgur.com/kxzQG.png" rel="noreferrer"><img src="https://i.stack.imgur.com/kxzQG.png" alt="enter image description here"></a></p>
  
  <p>Unfortunately, that's the entirely of the collection view. Scrolling doesn't scroll through any additional items. I've uploaded the sample app to <a href="https://www.dropbox.com/sh/wp2y7g0suemzcs1/AABVKfTZq54J7riy6BR7Mhxha?dl=0" rel="noreferrer">https://www.dropbox.com/sh/wp2y7g0suemzcs1/AABVKfTZq54J7riy6BR7Mhxha?dl=0</a> in case that's useful. Any ideas why this is breaking on 10.13 for me?  </p>