UICollectionViewFlowLayout does not expand item spacing to fit width? - ios

Question

In the flow layout section of this UICollectionView guide in Apple documentation, under "Specifying the Space Between Items and Lines", we read:
During layout, the flow layout object adds items to the current line until there is not enough space left to fit an entire item. If the line is just big enough to fit an integral number of items with no extra space, then the space between the items would be equal to the minimum spacing. If there is extra space at the end of the line, the layout object increases the interitem spacing until the items fit evenly within the line boundaries, as shown in Figure 3-3. Increasing the spacing improves the overall look of the items and prevents large gaps at the end of each line.
(emphasis mine)
But this does not seem to be true. As of iOS 12.4 and iOS 13 beta 7, if you make a vertically scrolling collection view with small cells that don't fill the available width, the inter-item spacing does not expand from the minimum to fill the width. See this example with two sections of 5 items each, 20pt x 20pt, with minimum item spacing, minimum line spacing, and section inset all at 4 pt:
Is the documentation outdated? Or might I be doing something weird? This is a sample app that consists of nothing but a collection view whose delegate / data source uses vanilla UICollectionViewCells that just have a background color, and implements the sizing methods mentioned above, and nothing else.
Code:
#implementation ViewController
- (void)viewDidLoad {
[super viewDidLoad];
[self.collectionView registerClass:[UICollectionViewCell class] forCellWithReuseIdentifier:#"Cell"];
}
-(NSInteger)numberOfSectionsInCollectionView:(UICollectionView *)collectionView
{
return 2;
}
-(NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section
{
return 5;
}
-(__kindof UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath
{
UICollectionViewCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:#"Cell" forIndexPath:indexPath];
cell.backgroundColor = [UIColor lightGrayColor];
return cell;
}
-(CGSize)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout *)collectionViewLayout sizeForItemAtIndexPath:(NSIndexPath *)indexPath
{
return CGSizeMake(20, 20);
}
-(UIEdgeInsets)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout *)collectionViewLayout insetForSectionAtIndex:(NSInteger)section
{
return UIEdgeInsetsMake(4, 4, 4, 4);
}
-(CGFloat)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout *)collectionViewLayout minimumInteritemSpacingForSectionAtIndex:(NSInteger)section
{
return 4;
}
-(CGFloat)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout *)collectionViewLayout minimumLineSpacingForSectionAtIndex:(NSInteger)section
{
return 4;
}