diff --git a/api/test-current.txt b/api/test-current.txt
index cee560b0..9472ce2 100644
--- a/api/test-current.txt
+++ b/api/test-current.txt
@@ -118,8 +118,8 @@
     method @RequiresPermission(android.Manifest.permission.MANAGE_ACTIVITY_STACKS) public void resizePinnedStack(int, android.graphics.Rect, boolean);
     method @RequiresPermission(android.Manifest.permission.MANAGE_ACTIVITY_STACKS) public void resizeTask(int, android.graphics.Rect);
     method @RequiresPermission(android.Manifest.permission.MANAGE_ACTIVITY_STACKS) public void setDisplayToSingleTaskInstance(int);
-    method @RequiresPermission(android.Manifest.permission.MANAGE_ACTIVITY_STACKS) public void setTaskWindowingMode(int, int, boolean) throws java.lang.SecurityException;
-    method @RequiresPermission(android.Manifest.permission.MANAGE_ACTIVITY_STACKS) public void setTaskWindowingModeSplitScreenPrimary(int, int, boolean, boolean, android.graphics.Rect, boolean) throws java.lang.SecurityException;
+    method @RequiresPermission(android.Manifest.permission.MANAGE_ACTIVITY_STACKS) public boolean setTaskWindowingMode(int, int, boolean) throws java.lang.SecurityException;
+    method @RequiresPermission(android.Manifest.permission.MANAGE_ACTIVITY_STACKS) public boolean setTaskWindowingModeSplitScreenPrimary(int, int, boolean, boolean, android.graphics.Rect, boolean) throws java.lang.SecurityException;
     method @RequiresPermission(android.Manifest.permission.MANAGE_ACTIVITY_STACKS) public void startSystemLockTaskMode(int);
     method @RequiresPermission(android.Manifest.permission.MANAGE_ACTIVITY_STACKS) public void stopSystemLockTaskMode();
     method public static boolean supportsMultiWindow(android.content.Context);
diff --git a/core/java/android/app/ActivityTaskManager.java b/core/java/android/app/ActivityTaskManager.java
index 58bff7f..9ba56cf 100644
--- a/core/java/android/app/ActivityTaskManager.java
+++ b/core/java/android/app/ActivityTaskManager.java
@@ -182,12 +182,13 @@
      * @param taskId The id of the task to set the windowing mode for.
      * @param windowingMode The windowing mode to set for the task.
      * @param toTop If the task should be moved to the top once the windowing mode changes.
+     * @return Whether the task was successfully put into the specified windowing mode.
      */
     @RequiresPermission(android.Manifest.permission.MANAGE_ACTIVITY_STACKS)
-    public void setTaskWindowingMode(int taskId, int windowingMode, boolean toTop)
+    public boolean setTaskWindowingMode(int taskId, int windowingMode, boolean toTop)
             throws SecurityException {
         try {
-            getService().setTaskWindowingMode(taskId, windowingMode, toTop);
+            return getService().setTaskWindowingMode(taskId, windowingMode, toTop);
         } catch (RemoteException e) {
             throw e.rethrowFromSystemServer();
         }
@@ -208,13 +209,14 @@
      *                      docked stack. Pass {@code null} to use default bounds.
      * @param showRecents If the recents activity should be shown on the other side of the task
      *                    going into split-screen mode.
+     * @return Whether the task was successfully put into splitscreen.
      */
     @RequiresPermission(android.Manifest.permission.MANAGE_ACTIVITY_STACKS)
-    public void setTaskWindowingModeSplitScreenPrimary(int taskId, int createMode, boolean toTop,
+    public boolean setTaskWindowingModeSplitScreenPrimary(int taskId, int createMode, boolean toTop,
             boolean animate, Rect initialBounds, boolean showRecents) throws SecurityException {
         try {
-            getService().setTaskWindowingModeSplitScreenPrimary(taskId, createMode, toTop, animate,
-                    initialBounds, showRecents);
+            return getService().setTaskWindowingModeSplitScreenPrimary(taskId, createMode, toTop,
+                    animate, initialBounds, showRecents);
         } catch (RemoteException e) {
             throw e.rethrowFromSystemServer();
         }
diff --git a/core/java/android/app/IActivityTaskManager.aidl b/core/java/android/app/IActivityTaskManager.aidl
index 503f5c5..be2f144 100644
--- a/core/java/android/app/IActivityTaskManager.aidl
+++ b/core/java/android/app/IActivityTaskManager.aidl
@@ -231,8 +231,9 @@
      * @param taskId The id of the task to set the windowing mode for.
      * @param windowingMode The windowing mode to set for the task.
      * @param toTop If the task should be moved to the top once the windowing mode changes.
+     * @return Whether the task was successfully put into the specified windowing mode.
      */
-    void setTaskWindowingMode(int taskId, int windowingMode, boolean toTop);
+    boolean setTaskWindowingMode(int taskId, int windowingMode, boolean toTop);
     void moveTaskToStack(int taskId, int stackId, boolean toTop);
     /**
      * Resizes the input pinned stack to the given bounds with animation.
diff --git a/services/core/java/com/android/server/wm/ActivityTaskManagerService.java b/services/core/java/com/android/server/wm/ActivityTaskManagerService.java
index 5a5976b..5f3e3a3 100644
--- a/services/core/java/com/android/server/wm/ActivityTaskManagerService.java
+++ b/services/core/java/com/android/server/wm/ActivityTaskManagerService.java
@@ -2247,11 +2247,11 @@
     }
 
     @Override
-    public void setTaskWindowingMode(int taskId, int windowingMode, boolean toTop) {
+    public boolean setTaskWindowingMode(int taskId, int windowingMode, boolean toTop) {
         if (windowingMode == WINDOWING_MODE_SPLIT_SCREEN_PRIMARY) {
-            setTaskWindowingModeSplitScreenPrimary(taskId, SPLIT_SCREEN_CREATE_MODE_TOP_OR_LEFT,
+            return setTaskWindowingModeSplitScreenPrimary(taskId,
+                    SPLIT_SCREEN_CREATE_MODE_TOP_OR_LEFT,
                     toTop, ANIMATE, null /* initialBounds */, true /* showRecents */);
-            return;
         }
         enforceCallerIsRecentsOrHasPermission(MANAGE_ACTIVITY_STACKS, "setTaskWindowingMode()");
         synchronized (mGlobalLock) {
@@ -2261,7 +2261,7 @@
                         MATCH_TASK_IN_STACKS_ONLY);
                 if (task == null) {
                     Slog.w(TAG, "setTaskWindowingMode: No task for id=" + taskId);
-                    return;
+                    return false;
                 }
 
                 if (DEBUG_STACK) Slog.d(TAG_STACK, "setTaskWindowingMode: moving task=" + taskId
@@ -2278,6 +2278,7 @@
                     stack.moveToFront("setTaskWindowingMode", task);
                 }
                 stack.setWindowingMode(windowingMode);
+                return true;
             } finally {
                 Binder.restoreCallingIdentity(ident);
             }
@@ -2696,6 +2697,7 @@
      *                      stack. Pass {@code null} to use default bounds.
      * @param showRecents If the recents activity should be shown on the other side of the task
      *                    going into split-screen mode.
+     * @return Whether the task was successfully put into splitscreen.
      */
     @Override
     public boolean setTaskWindowingModeSplitScreenPrimary(int taskId, int createMode,
@@ -2705,20 +2707,26 @@
         synchronized (mGlobalLock) {
             final long ident = Binder.clearCallingIdentity();
             try {
+                if (isInLockTaskMode()) {
+                    Slog.w(TAG, "setTaskWindowingModeSplitScreenPrimary: Is in lock task mode="
+                            + getLockTaskModeState());
+                    return false;
+                }
+
                 final Task task = mRootWindowContainer.anyTaskForId(taskId,
                         MATCH_TASK_IN_STACKS_ONLY);
                 if (task == null) {
                     Slog.w(TAG, "setTaskWindowingModeSplitScreenPrimary: No task for id=" + taskId);
                     return false;
                 }
-                if (DEBUG_STACK) Slog.d(TAG_STACK,
-                        "setTaskWindowingModeSplitScreenPrimary: moving task=" + taskId
-                                + " to createMode=" + createMode + " toTop=" + toTop);
                 if (!task.isActivityTypeStandardOrUndefined()) {
                     throw new IllegalArgumentException("setTaskWindowingMode: Attempt to move"
                             + " non-standard task " + taskId + " to split-screen windowing mode");
                 }
 
+                if (DEBUG_STACK) Slog.d(TAG_STACK,
+                        "setTaskWindowingModeSplitScreenPrimary: moving task=" + taskId
+                                + " to createMode=" + createMode + " toTop=" + toTop);
                 mWindowManager.setDockedStackCreateStateLocked(createMode, initialBounds);
                 final int windowingMode = task.getWindowingMode();
                 final ActivityStack stack = task.getStack();