TabRail layout fixes (#6172)

* Attempt another TabRail layout fix by just keeping it visible all the time
* Move one of the sidebar dividers to the tab rail.

ElementX/Sources/Application/Navigation/NavigationCoordinators.swift

@@ -397,8 +397,7 @@ private struct NavigationSplitCoordinatorView: View {
             .environment(\.isInSidebar, true)
             // The tab rail's background tracks the detail module, so exclude the sidebar's background value.
             .transformPreference(CompoundBackgroundPreferenceKey.self) { $0 = nil }
-            .overlay(alignment: .leading) { sidebarDivider }
+            .overlay(alignment: .trailing) { NavigationSidebarDivider() }
-            .overlay(alignment: .trailing) { sidebarDivider }
         } detail: {
             if let detailModule = navigationSplitCoordinator.detailModule {
                 detailModule.coordinator?.toPresentable()
@@ -416,9 +415,11 @@ private struct NavigationSplitCoordinatorView: View {
         .animation(.elementDefault, value: navigationSplitCoordinator.sidebarModule)
         .animation(.noAnimation, value: navigationSplitCoordinator.detailModule) // Don't crossfade the detail transition on iPad.
     }
+}
 
-    @ViewBuilder
+/// The vertical divider that separates the sidebar from its neighbours on iOS 27, where the sidebar is no longer inset.
-    var sidebarDivider: some View {
+struct NavigationSidebarDivider: View {
+    var body: some View {
         if #available(iOS 27, *) {
             HStack(spacing: 0) { // The stack is necessary for the divider's orientation.
                 Divider()

ElementX/Sources/Application/Navigation/NavigationTabCoordinator.swift

@@ -307,48 +307,6 @@ extension EnvironmentValues {
     @Entry var tabViewHorizontalSizeClass: UserInterfaceSizeClass?
 }
 
-/// Tracks whether the tab rail should be shown alongside the split view's sidebar.
-///
-/// The rail is laid out beside the split view, so hiding it hands that width back to the split, which may
-/// then have room for its sidebar again, which would show the rail, ad infinitum. The split reports the
-/// same column visibility whether it collapsed on its own or because the sidebar was toggled, and at the
-/// same width, so the two are told apart by when they arrive: the rebound lands in the next layout pass.
-private struct TabRailVisibility {
-    /// The longest a collapse can follow the rail being shown and still be considered caused by it.
-    private static let reboundDuration: TimeInterval = 0.5
-    
-    private(set) var isVisible = true
-    
-    private var containerWidth: CGFloat = 0
-    /// The container width at which the rail and the sidebar have been shown not to fit together.
-    private var blockedWidth: CGFloat?
-    /// When the rail was last shown, used to spot the split view collapsing as a direct result of it.
-    private var shownDate = Date.distantPast
-    
-    mutating func containerWidthChanged(to width: CGFloat) {
-        guard width != containerWidth else { return }
-        
-        containerWidth = width
-        blockedWidth = nil // The rail and the sidebar may well fit at the new width.
-    }
-    
-    mutating func splitVisibilityChanged(to visibility: NavigationSplitViewVisibility) {
-        if visibility == .detailOnly {
-            // A collapse this soon after showing the rail was caused by the rail's own width, so showing
-            // it again would only collapse the split view again, flickering forever. Note that the two
-            // don't fit at this width and leave the rail hidden until that changes.
-            if shownDate.timeIntervalSinceNow > -Self.reboundDuration {
-                blockedWidth = containerWidth
-            }
-            
-            isVisible = false
-        } else if !isVisible, blockedWidth != containerWidth {
-            isVisible = true
-            shownDate = .now
-        }
-    }
-}
-
 private struct NavigationTabCoordinatorView<Tag: Hashable>: View {
     @Environment(\.horizontalSizeClass) private var horizontalSizeClass
     
@@ -357,7 +315,6 @@ private struct NavigationTabCoordinatorView<Tag: Hashable>: View {
     @State private var standardAppearance = UITabBarAppearance()
     @State private var window: UIWindow?
     @State private var isFullScreen = true
-    @State private var railVisibility = TabRailVisibility()
     @State private var railBackgroundColor: Color = .compound.bgCanvasDefault
     
     var body: some View {
@@ -394,20 +351,16 @@ private struct NavigationTabCoordinatorView<Tag: Hashable>: View {
         }
     }
     
-    /// The column visibility of the split coordinator for the currently selected tab, if any.
-    private var selectedTabSplitColumnVisibility: NavigationSplitViewVisibility {
-        let selectedTabDetails = navigationTabCoordinator.tabModules
-            .first { $0.details.tag == navigationTabCoordinator.selectedTab }?.details
-        return selectedTabDetails?.navigationSplitCoordinator?.columnVisibility ?? .all
-    }
-    
     var tabRailLayout: some View {
+        // The rail is always shown. Removing it hands its width back to the split view, which then has
+        // room for its sidebar again, which shows the rail, ad infinitum. Keeping it means the split's
+        // width never depends on it and the two can't drive each other.
         HStack(spacing: 0) {
-            if railVisibility.isVisible {
+            TabRailView(navigationTabCoordinator: navigationTabCoordinator, isFullScreen: isFullScreen)
-                TabRailView(navigationTabCoordinator: navigationTabCoordinator, isFullScreen: isFullScreen)
+                .background(railBackgroundColor.ignoresSafeArea())
-                    .background(railBackgroundColor.ignoresSafeArea())
+                .animation(.easeInOut(duration: 0.4).disabledDuringTests(), value: railBackgroundColor)
-                    .animation(.easeInOut(duration: 0.4).disabledDuringTests(), value: railBackgroundColor)
+                .overlay(alignment: .trailing) { NavigationSidebarDivider() }
-            }
+                .zIndex(1)
             
             if let module = navigationTabCoordinator.tabModules.first(where: { $0.details.tag == navigationTabCoordinator.selectedTab }) {
                 module.coordinator?.toPresentable()
@@ -428,26 +381,14 @@ private struct NavigationTabCoordinatorView<Tag: Hashable>: View {
             }
         }
         .onGeometryChange(for: CGSize.self) { geometry in
-            geometry.size
+            geometry.size // We don't need the size, but it's a signal to re-compute.
-        } action: { size in
+        } action: { _ in
-            railVisibility.containerWidthChanged(to: size.width)
-            
             guard let newValue = window?.isFullScreen else { return }
             
             if newValue != isFullScreen {
                 isFullScreen = newValue
             }
         }
-        .onChange(of: selectedTabSplitColumnVisibility, initial: true) {
-            // The rail consumes width, which feeds back into the split's column visibility (not the
-            // same as flattening everything into the single stack), creating an observation tracking
-            // feedback loop that locks up the app while resizing.
-            //
-            // Use a stored value (instead of a computed value) that is updated on the next run loop to break the loop.
-            DispatchQueue.main.async {
-                railVisibility.splitVisibilityChanged(to: selectedTabSplitColumnVisibility)
-            }
-        }
     }
     
     var tabViewLayout: some View {
@@ -489,14 +430,6 @@ struct TabRailView<Tag: Hashable>: View {
         isFullScreen || ProcessInfo.processInfo.isiOSAppOnMac ? 0 : 48
     }
     
-    var trailingPadding: CGFloat {
-        if #available(iOS 27, *) {
-            8
-        } else {
-            0
-        }
-    }
-    
     var body: some View {
         ScrollView {
             VStack(spacing: 16) {
@@ -516,8 +449,7 @@ struct TabRailView<Tag: Hashable>: View {
                     .badge(10) // TODO: Check if this works.
                 }
             }
-            .padding(.leading, 8)
+            .padding(.horizontal, 8)
-            .padding(.trailing, trailingPadding)
             .padding(.top, topPadding)
             .padding(.bottom)
         }