Make sure the Tab Rail's background matches the split view's detail background. (#6003)

compound-ios/Sources/Compound/Colors/CompoundBackground.swift

@@ -0,0 +1,35 @@
+//
+// Copyright 2026 Element Creations Ltd.
+//
+// SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial.
+// Please see LICENSE files in the repository root for full details.
+//
+
+import SwiftUI
+
+/// The reported background drawn by a view.
+public enum CompoundBackground: Equatable {
+    /// A specific background colour.
+    case color(Color)
+    /// The default canvas background.
+    case `default`
+    
+    public var colorValue: Color {
+        switch self {
+        case .color(let color): color
+        case .default: .compound.bgCanvasDefault
+        }
+    }
+}
+
+/// A background reported up the hierarchy by a view that opts in, so that surrounding chrome (e.g. the
+/// tab rail that sits alongside iOS 26's inset sidebar) can match that view's canvas.
+public struct CompoundBackgroundPreferenceKey: PreferenceKey {
+    public static let defaultValue: CompoundBackground? = nil
+    
+    public static func reduce(value: inout CompoundBackground?, nextValue: () -> CompoundBackground?) {
+        if let nextValue = nextValue() {
+            value = nextValue
+        }
+    }
+}

compound-ios/Sources/Compound/List/ListStyles.swift

@@ -24,11 +24,13 @@ public extension View {
             environment(\.defaultMinListRowHeight, 48)
                 .scrollContentBackground(.hidden)
                 .background(Color.compound.bgSubtleSecondaryLevel0.ignoresSafeArea())
+                .preference(key: CompoundBackgroundPreferenceKey.self, value: .color(.compound.bgSubtleSecondaryLevel0))
         case .plain:
             listStyle(.plain)
                 .environment(\.defaultMinListRowHeight, 48)
                 .scrollContentBackground(.hidden)
                 .background(Color.compound.bgCanvasDefault.ignoresSafeArea())
+                .preference(key: CompoundBackgroundPreferenceKey.self, value: .default)
         }
     }