apply empty transaction in startActivitySync

Force SurfaceFlinger to update synchronously by applying an empty
transaction. Without this, SurfaceFlinger will only update after the
next vsync, which may only happen after the instrumentation already

Since InputFlinger will only have the proper InputWindowInfo after
SurfaceFlinger updates, waiting for the vsync caused some
instruemntation tests to be flaky.

Bug: 138263890
Test: atest android.view.cts.HoverTest
Change-Id: I5457ab67ac574530dc1aa84549ca11e7e3f0d714
(cherry picked from commit 03dde38efcb3c06c6317f9fcf1265e37a66811d9)
diff --git a/core/java/android/app/ b/core/java/android/app/
index 41733b3..9720e9f 100644
--- a/core/java/android/app/
+++ b/core/java/android/app/
@@ -50,6 +50,7 @@
 import android.view.KeyCharacterMap;
 import android.view.KeyEvent;
 import android.view.MotionEvent;
+import android.view.SurfaceControl;
 import android.view.ViewConfiguration;
 import android.view.Window;
 import android.view.WindowManagerGlobal;
@@ -528,6 +529,12 @@
             } while (mWaitingActivities.contains(aw));
+            // Apply an empty transaction to ensure SF has a chance to update before
+            // the Activity is ready (b/138263890).
+            try (SurfaceControl.Transaction t = new SurfaceControl.Transaction()) {
+                t.apply(true);
+            }
             return aw.activity;