The previous algorithm was buggy and did not handle the case when multiple VCs are being dismissed. Unfortunately I couldn't find a reliable way that'd allow for reshuffling modally presented VCs (inserting or deleting VCs not from the top) and so I added a special check that'd throw in the case someone attemted to do this.
When more than one screen is initially mounted we were only installing the top navigator with UINavController. This was cauing the back-handling logic to not work properly.
iOS navbar largeTitle setting allows the navbar to be automatically collapsed when scrolling. For that to work iOS expects scrollable component to be rendered in the screen view hierarchy as a first children (or just to be looked up on the ancestor path using first children when navigating down). On top of that in RN we should use contentInsetAdjustmentBehavior="automatic" for SV's contentInsets to adjust as expected when the header scrolls and to allow for the SV background to be present below the navigation bar.
This works pretty well w/o any changes. However when testing I disovered one issue with the navigation bar rendering in a collapsed state on the first render. After that you could've scroll down to reveal large title bar but the initial state would be collapsed. As on iOS it is expected for large title enabled screens to initially render in the revealed state a workaround was required. It turned out that the header rendered in collapsed state because at the moment when navigation controller is created is has an empty list of view controllers. The list is empty because react first creates nnavigation controller and only then adds children to it. It turned out the fix was to populate viewControllers with a single item at first, then when controllers list is replaced with the children provided from react the header renders properly in the revealed state.
This change fixes the problem when header would update to the previous screen configuration as a result of starting swipe back gesture but never restore to the original one in case swipe back gets cancelled (e.g. user didn't swipe far enough). The problem was that as a result of swipe back we'd apply header config changes but after cancel there was no trigger to reset header to the previous state.
On iOS by default modals show up in full-screen mode. This behavior can be customized and instead of mouting new screens in top level window they can be mounted under a given UINavController. We used to be relying on that behavior (see "CurrentContext" presentation mode). This, apparently wasn't matching the default functionality of the OS. This change is adding it as a default and keeping the old way under newly exposed presentation modes: containedModal and containedTransparentModal
Thanks to this change, iOS 13 modals can work properly.
