Fixes UI freezes when multiple Flutter VC shared one engine (#18816)

Co-authored-by: Aaron Clarke <[email protected]>

Author: zhongwuzw

shell/platform/darwin/ios/framework/Source/FlutterViewController.mm

@@ -553,9 +553,11 @@ - (void)viewWillDisappear:(BOOL)animated {
 
 - (void)viewDidDisappear:(BOOL)animated {
   TRACE_EVENT0("flutter", "viewDidDisappear");
-  [self surfaceUpdated:NO];
-  [[_engine.get() lifecycleChannel] sendMessage:@"AppLifecycleState.paused"];
-  [self flushOngoingTouches];
+  if ([_engine.get() viewController] == self) {
+    [self surfaceUpdated:NO];
+    [[_engine.get() lifecycleChannel] sendMessage:@"AppLifecycleState.paused"];
+    [self flushOngoingTouches];
+  }
 
   [super viewDidDisappear:animated];
 }

shell/platform/darwin/ios/framework/Source/FlutterViewControllerTest.mm

@@ -54,11 +54,45 @@ - (UIAccessibilityContrast)accessibilityContrast;
 #endif
 
 @interface FlutterViewController (Tests)
+- (void)surfaceUpdated:(BOOL)appeared;
 - (void)performOrientationUpdate:(UIInterfaceOrientationMask)new_preferences;
 @end
 
 @implementation FlutterViewControllerTest
 
+- (void)testViewDidDisappearDoesntPauseEngine {
+  id engine = OCMClassMock([FlutterEngine class]);
+  id lifecycleChannel = OCMClassMock([FlutterBasicMessageChannel class]);
+  OCMStub([engine lifecycleChannel]).andReturn(lifecycleChannel);
+  FlutterViewController* viewControllerA = [[FlutterViewController alloc] initWithEngine:engine
+                                                                                 nibName:nil
+                                                                                  bundle:nil];
+  FlutterViewController* viewControllerB = [[FlutterViewController alloc] initWithEngine:engine
+                                                                                 nibName:nil
+                                                                                  bundle:nil];
+  id viewControllerMock = OCMPartialMock(viewControllerA);
+  OCMStub([viewControllerMock surfaceUpdated:NO]);
+  OCMStub([engine viewController]).andReturn(viewControllerB);
+  [viewControllerA viewDidDisappear:NO];
+  OCMReject([lifecycleChannel sendMessage:@"AppLifecycleState.paused"]);
+  OCMReject([viewControllerMock surfaceUpdated:[OCMArg any]]);
+}
+
+- (void)testViewDidDisappearDoesPauseEngine {
+  id engine = OCMClassMock([FlutterEngine class]);
+  id lifecycleChannel = OCMClassMock([FlutterBasicMessageChannel class]);
+  OCMStub([engine lifecycleChannel]).andReturn(lifecycleChannel);
+  FlutterViewController* viewController = [[FlutterViewController alloc] initWithEngine:engine
+                                                                                nibName:nil
+                                                                                 bundle:nil];
+  id viewControllerMock = OCMPartialMock(viewController);
+  OCMStub([viewControllerMock surfaceUpdated:NO]);
+  OCMStub([engine viewController]).andReturn(viewController);
+  [viewController viewDidDisappear:NO];
+  OCMVerify([lifecycleChannel sendMessage:@"AppLifecycleState.paused"]);
+  OCMVerify([viewControllerMock surfaceUpdated:NO]);
+}
+
 - (void)testBinaryMessenger {
   id engine = OCMClassMock([FlutterEngine class]);
   FlutterViewController* vc = [[FlutterViewController alloc] initWithEngine:engine