How to push a portrait-only UIViewController onto a landscape UINavigationController

push.m

// How to push a portrait-only UIViewController onto a landscape UINavigationController

- (void) _presentForcedVerticalNormalViewController:(UIViewController *)viewController animated:(BOOL)animated
{
    UIView *(^makeSnapshotOfViewController)(UIViewController *) = ^(UIViewController *vc) {
        UIView *result = [[vc view] snapshotViewAfterScreenUpdates:NO];
        [[_rootViewController view] addSubview:result];
        [result setFrame:[[vc view] bounds]];
        
        [result setUserInteractionEnabled:NO];
        
        return result;
    };

    // Manipulating the view controller hierarchy in completion blocks is dangerous and results in
    // exceptions.  This postpones the block for one cycle.
    //
    void (^then)(dispatch_block_t) = ^(dispatch_block_t block) {
        dispatch_async(dispatch_get_main_queue(), block);
    };

    NavigationController *fakeNavigationController = [[NavigationController alloc] initWithNavigationBarClass:[NavigationBar class] toolbarClass:nil];
    [fakeNavigationController setViewControllers:@[ viewController ]];
    [fakeNavigationController setDelegate:self];

    // As soon as we present fakeNavigationController, UIKit will attempt to redraw the ToC vc
    // in landscape orientation.  This looks bad, snapshot1 hides the glitch
    //
    UIView *snapshot1 = makeSnapshotOfViewController(_rootViewController);

    [_rootViewController presentViewController:fakeNavigationController animated:YES completion:^{
        [UIViewController attemptRotationToDeviceOrientation];
        [snapshot1 removeFromSuperview];
        
        then(^{
            // snapshot2 prevents the ToC vc from showing up while we are dismissing the modal
            // fakeNavigationController and pushing the original viewController
            //
            UIView *snapshot2 = makeSnapshotOfViewController(fakeNavigationController);
            
            [_rootViewController dismissViewControllerAnimated:NO completion:^{
                then(^{
                    [_rootViewController pushViewController:viewController animated:NO];
                    
                    then(^{
                        [UIView animateWithDuration:0.1 animations:^{
                            [snapshot2 setAlpha:0.0];
                        } completion:^(BOOL finished) {
                            [snapshot2 removeFromSuperview];
                        }];
                    });
                });
            }];
        });
    }];
}