To sum up: The vanishing vertical tab bar was fixed by setting its background color to clear: it appears that (when a tab bar button is not pressed) the background renders on top of the tab bar items, not behind them. Setting my views‘ self.edgesForExtendedLayout = UIRectEdgeAll not only made my views render edge-to-edge, it made my safe area insets start working to keep my content out from under the nav bar automatically, with no additional fiddling on my part — and my content now works perfectly with the upscrolling nav bar on the Duo.

Update: added the following to my views‘ viewDidLoad: if (@available(iOS 18.0, *)) { self.edgesForExtendedLayout = UIRectEdgeAll; self.extendedLayoutIncludesOpaqueBars = YES; } ...and now all the views extend full-screen, edge-to-edge on all devices in all modes. (And, yes, I should have done this when iOS 18 came out instead of fiddling with my views‘ frames myself! I'm guessing the 58-pixel-high view in the screenshot above is a pathology that no one who has previously updated their app properly for iOS 18+ would ever see!)

If you mean that white bit above your blue nav bar, that's the top of the underlying view (at least, in my app that's what it is). My app uses UIViews that “fill their space", managed by a TabViewController. If I don't do anything to them in my code, they normally fill the space between the nav bar at the top of the screen and the tab bar at the bottom. However, in vertical-tab-bar modes on the Duo (and only on this device in this mode), the view starts at y=0, the top of the screen: will layout CowViewController: top 0 height 58, window height 678 (And as you can see, its height is only 58 pixels, for no reason that I can immediately determine, as the tab bar's top is at Y=0, too. I wind up overriding my view's frame to fix this.) Try setting your view's background color to something weird (like I did) to see if that's what's happening in your case, and setting the nav bar's background to a semitransparent color to be able to see through it.

Update: the vanishing vertical toolbar was fixed by setting the tab bar's background color to clear: self.tabBarController.tabBar.backgroundColor = [UIColor clearColor]; At least in the sim, the tab bar background is rendering on top of the foreground elements, not behind them! As for the nav bar, setting the background color to clear does work (I hadn't set it correctly before due to a bug in my code), which eliminates the fixed white bar, but there's still a nasty blurred translucent region that comes and goes (sometimes fading in and out, other times popping on and off), whenever the title is visible: At least I can set my scrolling view to be below the nav bar, so the blurred region never covers my content, but the scrolling title is, shall we say, sub-optimal. So I'm still curious if that title animation can be turned off, so that the title stays onscreen regardless of the underlying view's scrolling.