message: Stopped the nav stacks sharing the real estate
  Each navigation stack only had half of the view height. Add another stack and they'd only get a third each! Added stack children to shadow view - trick copied from RCTModalHostViewManager - so they're sized based on whole screen size
diff: #import "NVNavigationStackManager.h"
  #import "NVNavigationStackView.h"
  +#import <React/RCTShadowView.h>
  +
  +@interface NVNavigationStackShadowView : RCTShadowView
  +
  +@end
  +
  +@implementation NVNavigationStackShadowView
  +
  +- (void)insertReactSubview:(id<RCTComponent>)subview atIndex:(NSInteger)atIndex
  +{
  + [super insertReactSubview:subview atIndex:atIndex];
  + if ([subview isKindOfClass:[RCTShadowView class]]) {
  + ((RCTShadowView *)subview).size = RCTScreenSize();
  + }
  +}
  +
  +@end
  +
  @implementation NVNavigationStackManager
  RCT_EXPORT_MODULE()
  @@ -10,6 +28,11 @@ RCT_EXPORT_MODULE()
  return [[NVNavigationStackView alloc] init];
  }
  +- (RCTShadowView *)shadowView
  +{
  + return [NVNavigationStackShadowView new];
  +}
  +
  RCT_EXPORT_VIEW_PROPERTY(mostRecentEventCount, NSInteger)
  RCT_EXPORT_VIEW_PROPERTY(onDidNavigateBack, RCTBubblingEventBlock)
  