Inject tab drag capability registries

Sources/Bonsplit/Internal/Controllers/SplitViewController.swift

@@ -101,7 +101,7 @@ final class SplitViewController {
     convenience init(rootNode: SplitNode? = nil) {
         self.init(
             rootNode: rootNode,
-            tabDragTransferRegistry: .process
+            tabDragTransferRegistry: TabDragTransferRegistry()
         )
     }
 

Sources/Bonsplit/Public/BonsplitController.swift

@@ -115,15 +115,30 @@ public final class BonsplitController {
 
     // MARK: - Initialization
 
-    /// Create a new controller with the specified configuration
+    /// Creates an isolated controller with the specified configuration.
+    ///
+    /// Use ``init(configuration:tabDragTransferRegistry:)`` when multiple
+    /// controllers may exchange tabs.
+    ///
+    /// - Parameter configuration: The controller's behavior and appearance.
     public init(configuration: BonsplitConfiguration = .default) {
         self.configuration = configuration
         self.internalController = SplitViewController()
         configureInternalController()
     }
 
-    init(
-        configuration: BonsplitConfiguration = .default,
+    /// Creates a controller with explicit ownership of its tab-drag capabilities.
+    ///
+    /// Controllers that exchange tabs must receive the same registry. Controllers
+    /// created without one are isolated, which keeps independent hosts and tests
+    /// from sharing hidden process state.
+    ///
+    /// - Parameters:
+    ///   - configuration: The controller's behavior and appearance.
+    ///   - tabDragTransferRegistry: The capability registry shared by controllers
+    ///     that may exchange tabs.
+    public init(
+        configuration: BonsplitConfiguration,
         tabDragTransferRegistry: TabDragTransferRegistry
     ) {
         self.configuration = configuration

Sources/Bonsplit/Public/TabDragTransferRegistry.swift

@@ -5,12 +5,6 @@ import UniformTypeIdentifiers
 /// Process-local capability store shared by tab-drag sources and destinations.
 @MainActor
 public final class TabDragTransferRegistry {
-    /// The registry used by Bonsplit controllers created with the default initializer.
-    ///
-    /// AppKit exposes one drag pasteboard for the process, so the default registry
-    /// is process-wide as well. Construct a separate registry for isolated tests.
-    public static let process = TabDragTransferRegistry()
-
     /// The AppKit pasteboard type that carries opaque tab-drag capabilities.
     public static let pasteboardType = NSPasteboard.PasteboardType(
         UTType.tabTransfer.identifier

Tests/BonsplitTests/TabDragTransferRegistryTests.swift

@@ -27,9 +27,9 @@ struct TabDragTransferRegistryTests {
 
     @Test("A registered capability routes across controllers")
     func registeredCapabilityRoutesAcrossControllers() throws {
-        let sourceController = makeController()
-        let targetController = makeController()
-        let registry = sourceController.internalController.tabDragTransferRegistry
+        let registry = TabDragTransferRegistry()
+        let sourceController = makeController(registry: registry)
+        let targetController = makeController(registry: registry)
         let sourcePane = try #require(sourceController.internalController.focusedPane)
         let sourceTab = try #require(sourcePane.selectedTab)
         let targetPane = try #require(targetController.internalController.focusedPane)
@@ -53,6 +53,17 @@ struct TabDragTransferRegistryTests {
         #expect(receivedRequest?.sourcePaneId == sourcePane.id)
     }
 
+    @Test("Controllers do not share an implicit capability registry")
+    func defaultControllersHaveIsolatedCapabilityRegistries() {
+        let firstController = makeController()
+        let secondController = makeController()
+
+        #expect(
+            firstController.internalController.tabDragTransferRegistry
+                !== secondController.internalController.tabDragTransferRegistry
+        )
+    }
+
     @Test("An unregistered capability is rejected")
     func unregisteredCapabilityIsRejected() throws {
         let registry = TabDragTransferRegistry()