Simplify guards to match RemovingTransitionContext pattern and fix test

- Remove hasNotifiedCompletion flag, use isInvalidated guard instead
- Use XCTestExpectation for async batch transition test (crossDissolve)
- Place stack in UIWindow so animation completes properly

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: muukii

Sources/FluidStack/Transition/BatchRemovingTransitionContext.swift

@@ -36,7 +36,6 @@ public final class BatchRemovingTransitionContext: TransitionContext {
 
   private let onCompleted: (BatchRemovingTransitionContext) -> Void
   private var childContexts: [ChildContext] = []
-  private var hasNotifiedCompletion: Bool = false
   private var callbacks: [(CompletionEvent) -> Void] = []
 
   init(
@@ -92,9 +91,8 @@ public final class BatchRemovingTransitionContext: TransitionContext {
   /// Triggers ``addCompletionEventHandler(_:)`` with ``TransitionContext/CompletionEvent/interrupted``
   override func invalidate() {
     assert(Thread.isMainThread)
+    guard isInvalidated == false else { return }
     isInvalidated = true
-    guard hasNotifiedCompletion == false else { return }
-    hasNotifiedCompletion = true
     callbacks.forEach { $0(.interrupted) }
   }
 
@@ -110,9 +108,6 @@ public final class BatchRemovingTransitionContext: TransitionContext {
    Triggers ``addCompletionEventHandler(_:)`` with ``TransitionContext/CompletionEvent/succeeded``
    */
   func transitionSucceeded() {
-    assert(Thread.isMainThread)
-    guard hasNotifiedCompletion == false else { return }
-    hasNotifiedCompletion = true
     callbacks.forEach { $0(.succeeded) }
   }
 }

Tests/FluidStackTests/FluidStackControllerTests.swift

@@ -566,13 +566,14 @@ final class FluidStackControllerTests: XCTestCase {
   }
 
   /// Tests that batch removing completion is called exactly once.
-  /// Previously, when multiple StackingPlatterViews shared the same
-  /// BatchRemovingTransitionContext, each swapTransitionContext call
-  /// would fire invalidate() on the shared context, causing completion
-  /// to be called N times.
+  /// When fluidPop triggers batch removing (VC is not on top),
+  /// the completion must fire exactly once after the batch transition completes.
   func testBatchRemovingCompletionCalledOnce() {
 
+    let window = UIWindow()
     let stack = FluidStackController()
+    window.rootViewController = stack
+    window.makeKeyAndVisible()
 
     let vc1 = UIViewController()
     let vc2 = UIViewController()
@@ -586,17 +587,16 @@ final class FluidStackControllerTests: XCTestCase {
 
     XCTAssertEqual(stack.stackingViewControllers.count, 4)
 
+    let exp = expectation(description: "completion called")
+    exp.assertForOverFulfill = true
     var completionCallCount = 0
 
-    // Remove vc2, which is not on top -> triggers batch removing of [vc2, vc3, vc4]
-    stack.removeViewController(
-      vc2,
-      transition: nil,
-      transitionForBatch: .disabled,
-      completion: { event in
-        completionCallCount += 1
-      }
-    )
+    vc2.fluidPop { _ in
+      completionCallCount += 1
+      exp.fulfill()
+    }
+
+    wait(for: [exp], timeout: 2)
 
     XCTAssertEqual(stack.stackingViewControllers.count, 1)
     XCTAssertEqual(completionCallCount, 1, "Completion should be called exactly once")