Fix flakiness in FlutterVSyncWaiterTest.VSyncWorks and FlutterDisplayLinkTest.WorkaroundForFB13482573 (#51405)

- Removes potential source of flakiness where the tests assumes that
scheduled block will be performed within certain time period, which,
despite the tolerances may not be the case on test runner.

- Ensures that `viewDidChangeWindow` notification is not received while
invalidating the displayLink and removing the display link view, which
could deadlock at the end of `WorkaroundForFB13482573` test because the
notification would come while `FlutterDisplayLink` is in `@synchronized`
block.

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Author: knopp

shell/platform/darwin/macos/framework/Source/FlutterDisplayLink.mm

@@ -262,8 +262,11 @@ - (instancetype)initWithView:(NSView*)view {
 - (void)invalidate {
   @synchronized(self) {
     FML_DCHECK([NSThread isMainThread]);
-    [_view removeFromSuperview];
+    // Unregister observer before removing the view to ensure
+    // that the viewDidChangeWindow notification is not received
+    // while in @synchronized block.
     [[NSNotificationCenter defaultCenter] removeObserver:self];
+    [_view removeFromSuperview];
     _view = nil;
     _delegate = nil;
   }

shell/platform/darwin/macos/framework/Source/FlutterVSyncWaiterTest.mm

@@ -140,7 +140,14 @@ static void BusyWait(CFTimeInterval duration) {
                       CFRunLoopStop(CFRunLoopGetCurrent());
                     }];
 
+  __block CFTimeInterval expectedStartUntil;
+  // Warm up tick is scheduled immediately in a scheduled block. Schedule another
+  // block here to determine the maximum time when the warm up tick should be
+  // scheduled.
   [waiter waitForVSync:kWarmUpBaton];
+  [[NSRunLoop currentRunLoop] performBlock:^{
+    expectedStartUntil = CACurrentMediaTime();
+  }];
 
   // Reference vsync to setup phase.
   CFTimeInterval now = CACurrentMediaTime();
@@ -166,15 +173,22 @@ static void BusyWait(CFTimeInterval duration) {
   // Vsync without baton should pause the display link.
   [displayLink tickWithTimestamp:now + 3.5 * displayLink.nominalOutputRefreshPeriod
                  targetTimestamp:now + 5 * displayLink.nominalOutputRefreshPeriod];
-  // Make sure to run the timer scheduled in display link callback.
-  CFRunLoopRunInMode(kCFRunLoopDefaultMode, 0.02, NO);
+
+  CFTimeInterval start = CACurrentMediaTime();
+  while (!displayLink.paused) {
+    // Make sure to run the timer scheduled in display link callback.
+    CFRunLoopRunInMode(kCFRunLoopDefaultMode, 0.02, NO);
+    if (CACurrentMediaTime() - start > 1.0) {
+      break;
+    }
+  }
   ASSERT_TRUE(displayLink.paused);
 
   EXPECT_EQ(entries.size(), size_t(4));
 
   // Warm up frame should be presented as soon as possible.
-  EXPECT_TRUE(fabs(entries[0].timestamp - now) < 0.005);
-  EXPECT_TRUE(fabs(entries[0].targetTimestamp - now) < 0.005);
+  EXPECT_TRUE(entries[0].timestamp <= expectedStartUntil);
+  EXPECT_TRUE(entries[0].targetTimestamp <= expectedStartUntil);
   EXPECT_EQ(entries[0].baton, kWarmUpBaton);
 
   EXPECT_DOUBLE_EQ(entries[1].timestamp, now + displayLink.nominalOutputRefreshPeriod);