Add appear test case

Author: Kyle-Ye

Sources/OpenSwiftUI/Core/Update/Update.swift

@@ -71,9 +71,24 @@ enum Update {
 
     @inline(__always)
     static func dispatchActions() {
-        // FIXME
-        for action in actions {
-            action()
+        guard !actions.isEmpty else {
+            return
+        }
+
+        let actions = Update.actions
+        Update.actions = []
+        performOnMainThread {
+            // TODO: Signpost.postUpdateActions
+            begin()
+            for action in actions {
+                let oldDepth = depth
+                action()
+                let newDepth = depth
+                if newDepth != oldDepth {
+                    fatalError("Action caused unbalanced updates.")
+                }
+            }
+            end()
         }
     }
 
@@ -107,5 +122,5 @@ extension Update {
 // FIXME: migrate to use @_extern(c, "xx") in Swift 6
 extension MovableLock {
     @_silgen_name("_MovableLockSyncMain")
-    static func syncMain(lock: MovableLock ,body: @escaping () -> Void)
+    static func syncMain(lock: MovableLock, body: @escaping () -> Void)
 }

Tests/OpenSwiftUICompatibilityTests/View/Modifier/AppearanceActionModifierTests.swift

@@ -0,0 +1,33 @@
+//
+//  AppearanceActionModifierTests.swift
+//  OpenSwiftUICompatibilityTests
+
+import Testing
+
+#if canImport(Darwin)
+struct AppearanceActionModifierTests {
+    @Test
+    func appear() async throws {
+        struct ContentView: View {
+            var confirmation: Confirmation
+
+            var body: some View {
+                AnyView(EmptyView())
+                    .onAppear {
+                        confirmation()
+                    }
+            }
+        }
+
+        #if os(iOS)
+        await confirmation { @MainActor confirmation in
+            let vc = UIHostingController(rootView: ContentView(confirmation: confirmation))
+            vc.triggerLayout()
+            workaroundIssue87(vc)
+        }
+        #endif
+    }
+    
+    // TODO: Add disappear support and test case
+}
+#endif