fix(push-demo): return notification completion on main thread

Signed-off-by: Tom Brow <tomb@block.xyz>

mobile/ios/Runner/PushNativeState.swift

@@ -16,7 +16,14 @@ final class BuzzOneShotCompletion {
     let completion = completion
     self.completion = nil
     lock.unlock()
-    completion?()
+    guard let completion else { return }
+    if Thread.isMainThread {
+      completion()
+    } else {
+      // Reply publication finishes on a URLSession callback queue, while the
+      // notification response completion crosses back into UIKit.
+      DispatchQueue.main.async(execute: completion)
+    }
   }
 }
 

mobile/ios/RunnerTests/BuzzCommunicationNotificationTests.swift

@@ -217,6 +217,24 @@ final class BuzzPushSnapshotEnrichmentTests: XCTestCase {
 }
 
 final class BuzzPushNotificationResponseTests: XCTestCase {
+  func testBackgroundCompletionReturnsToMainThreadExactlyOnce() {
+    let completed = expectation(description: "completion returned on main thread")
+    var completionCount = 0
+    let completion = BuzzOneShotCompletion {
+      XCTAssertTrue(Thread.isMainThread)
+      completionCount += 1
+      completed.fulfill()
+    }
+
+    DispatchQueue.global().async {
+      completion.call()
+      completion.call()
+    }
+
+    wait(for: [completed], timeout: 1)
+    XCTAssertEqual(completionCount, 1)
+  }
+
   func testValidReminderActionRoutesContextWithoutOpeningFlutter() {
     let context = BuzzPushReplyContext(
       eventID: "message-id",