iPhone Duo vertical bars

On the outer display and on the inner display in landscape, iPhone Duo moves controls that normally sit at the top and bottom to the side, keeping vertical space for content and controls within reach. Only the inner display in portrait keeps horizontal bars. Top to bottom the side stack is Dynamic Island, status bar, toolbar (with navigation buttons), tab bar. (Tech Talk 111462, 0:28; Tech Talk 111466, 5:07; HIG Designing for iPhone Duo › Vertical controls)

Read references/vertical-bars.md for the code; references/api-availability.md for which parts compile with the selected SDK — axisBehavior, toolbarVerticalEdge, compression behavior and the opt-out need the iOS 27.1 SDK, which ships with Xcode 27.1, so on that toolchain they are a deployment-target question rather than a blocked one; titles, symbols, badges, placements, visibilityPriority and ToolbarOverflowMenu are iOS 27.0 and work on either. verticalBarEdge has no UITrait class — observe it with registerForTraitChanges(UITraitCollection.systemTraitsAffectingVerticalBarEdge). Where the corner camera and the side controls physically sit: references/device-geometry.md.

How the system decides

• Only container bars participate. SwiftUI: .toolbar inside NavigationStack or NavigationSplitView. UIKit: UINavigationController and UITabBarController. Standalone UIToolbar, UINavigationBar, UITabBar content is not considered. (2:00)
• One shared region. Navigation, toolbar and tab bar controls stack into one vertical region. In split views only the detail column participates; inspectors get no bar of their own. The bar is aligned with the hardware and stays on the same side in right-to-left languages. (3:09)
• Sheets differ per display. On the outer display a sheet's toolbar goes vertical by default. On the inner display sheets are centered with horizontal bars; presentationPlacement(.leading) / .trailing (UIKit: sheetPresentationController?.preferredPlacement) moves a sheet to an edge, and only a trailing sheet receives a vertical bar. (3:58; Preparing your app for iPhone Duo › Optimize bars for vertical presentation)
• Keyboard accessory bars stay with the keyboard. (10:00)
• Fixed width, flexible height. Items with an icon go vertical; text-only items stay horizontal. The title is still required — it is used in overflow menus and expanded forms. (5:56)
• Custom views stay horizontal by default. (8:00)
• Overflow runs bottom to top, and toolbars compress before the tab bar by default. Overflow happens sooner on the outer display in landscape, with the keyboard up, and with Picture in Picture pinned at the top in open portrait. (11:40, 13:10)

Audit checklist (per screen)

1. Container. Is every bar item owned by a navigation or tab container? Scanner rule DUO007 finds standalone bars; move their items into the container.
2. Order. Top of the vertical bar: primary navigation (back or close), then prominent actions (done, save). Use .cancellationAction / leading item groups with leftItemsSupplementBackButton = false for close, and .topBarPinnedTrailing / pinnedTrailingGroup for prominent actions. Keep controls associated with the container they belong to. (4:29) Group related items with ToolbarItemGroup / UIBarButtonItemGroup instead of manual spacing, and keep controls next to the content they affect — Mail's list controls stay above the leading pane. (HIG › Vertical controls)
3. Title and symbol on every item. Provide both, even when only the symbol shows.
4. Text that carries information stays horizontal. A symbol plus redundant text → symbol only (use a badge for counts, iOS 26 badge API). Text with standalone value (a cart showing a total) → keep, and it stays in the horizontal bar. The system Edit button and wide controls such as segmented controls stay horizontal on their own. (9:00; 8:25; Tech Talk 111466, 6:20)
5. Axis for items that change shape. An item that switches between symbol and text (Select ↔ Done) → .horizontalOnly, and keep related items on the same axis. A custom view that has a vertical representation → .verticalPreferred. (8:00)
6. Custom views fit. Fixed bar width, or a vertically adapted layout keyed off toolbarVerticalEdge / verticalBarEdge. Flexible spacers collapse to zero vertically; fixed spacers keep their minimum. Legible with no scroll edge effect and with the Reduce Transparency background. (10:07)
7. Overflow. One overflow menu, and the ellipsis symbol belongs to it (scanner rule DUO011). A custom ellipsis menu is one of two things: a grab bag of secondary actions — move it into ToolbarOverflowMenu / additionalOverflowItems so people see one "more" instead of two — or a single named function such as Filter, which deserves its own symbol and title so it can survive in the vertical bar. (11:40)
8. Compression. Navigation-focused apps: default, or explicitly .prefersTabBar (toolbar compresses first). Task-oriented screens where actions matter more than tabs: .toolbarVerticalCompressionBehavior(.prefersToolbarItems) / verticalBarCompressionBehavior = .prefersBarItems. (12:23)
9. Priority. Set visibilityPriority on groups first, then items. Compose / New Note / primary creation: high. Status-bearing items (badged inbox): high, to preserve glanceability. Rare actions: low. Finer steps: ToolbarItemVisibilityPriority(higherThan:) / (lowerThan:); UIKit's default is .standard. (13:10)
10. Opt-out only where it fits. Single-page, bottom-heavy layouts (Calculator-like) and sheets with a single close button — the sheet then stops short of the camera and the status bar repositions (Tech Talk 111466, 9:03). Not as a fix for items that overflow. (14:21) The behavior resolves per window or presentation: a NavigationStack uses its top view, a TabView its selected tab, a NavigationSplitView its trailing-most column; make it a stable choice, never a toggle on a view's state, and use toolbarVisibility to hide bars instead. UIKit: preferredVerticalBarBehavior, childForPreferredVerticalBarBehavior, setNeedsUpdateOfVerticalBarConfiguration(). Otherwise don't override the default placement; a full-width layout suits only immersive, non-scrolling screens that don't conflict with the Dynamic Island or status bar. (HIG › Vertical controls; Tech Talk 111466, 6:33)
11. Backgrounds under the bar. A hero image or full-bleed header extends under the vertical bar with backgroundExtensionEffect() / UIBackgroundExtensionView (iOS 26) rather than a manually stretched frame; scrollable foreground content stays inset. (Preparing your app for iPhone Duo › Optimize bars for vertical presentation; Tech Talk 111466, 6:33)

Output

One recommendation per screen (not per item) in the format of references/recommendation-format.md, listing the checklist steps that apply. Items that need 27.1 APIs are marked blocked when the SDK lacks them; everything else can ship now and improves bars on every iPhone.

Verify

Pose matrix P1 (outer display: side controls), P2 (outer display landscape, keyboard up), P3 (inner landscape: vertical bar), P4 (inner portrait: horizontal), P11 (right-to-left), P12 (Reduce Transparency) from references/pose-test-matrix.md. Check which items overflow and in what order.