Collectionview change height

Hi,

I want to place a navigationbar on top of chatviewcontroller. When i place it programmatically collectionview which shows messages comes under navigationbar partly. When i try to change content inset of collectionview in viewDidLayoutSubviews collectionview comes to desired positions but then it doesnt scroll to bottom. Even when i send new messages it doesnt scroll to bottom. Any idea about changing collectionview position?

Below is the code that i use.

```
override func viewDidLayoutSubviews() {
super.viewDidLayoutSubviews()
self.collectionView.contentInset.top = 50;
}
```

Start with the reported viewDidLayoutSubviews implementation and the collectionView contentInset change, then reproduce the navigation-bar overlap and the lost bottom-scrolling behavior. Done means the collection view is positioned below the navigation bar while still scrolling to the latest message after layout and when sending new messages.

Written by the indexing model from the issue text.