Modern Collection Views and Readable Content Guide

If you want to inset your cells to the readable content width, you can use the contentInsetsReference API on UICollectionViewCompositionalLayoutConfiguration (to set a default value for the entire layout) or NSCollectionLayoutSection (to control the value per-section) to do this.

For list layouts (using UICollectionLayoutListConfiguration), you need to set both the contentInsetsReference and contentInsets on NSCollectionLayoutSection, because list sections do not use a default contentInsetsReference value of .automatic (meaning they won't inherit the value from the UICollectionViewCompositionalLayoutConfiguration) and they also may have nonzero default leading & trailing contentInsets which you probably want to remove.

Here is an example of how to do this using the NSCollectionLayoutSection API:

let layout = UICollectionViewCompositionalLayout { sectionIndex, layoutEnvironment in
    // Create the layout section using a list configuration.
    let listConfig = UICollectionLayoutListConfiguration(appearance: .insetGrouped)
    var section = NSCollectionLayoutSection.list(using: listConfig, layoutEnvironment: layoutEnvironment)
    
    // Change the section's content insets reference to the readable content.
    // This changes the way that the insets in the section's contentInsets property are interpreted.
    section.contentInsetsReference = .readableContent
    
    // Zero out the default leading/trailing contentInsets, but preserve the default top/bottom values.
    // This ensures each section will be inset horizontally exactly to the readable content width.
    var contentInsets = section.contentInsets
    contentInsets.leading = 0
    contentInsets.trailing = 0
    section.contentInsets = contentInsets
    
    return section
}