diff --git a/services/core/java/com/android/server/am/ActivityDisplay.java b/services/core/java/com/android/server/am/ActivityDisplay.java
index b11b16e12c3a3..2d2424fe7b590 100644
--- a/services/core/java/com/android/server/am/ActivityDisplay.java
+++ b/services/core/java/com/android/server/am/ActivityDisplay.java
@@ -65,6 +65,12 @@ class ActivityDisplay extends ConfigurationContainer<ActivityStack> {
     static final int POSITION_TOP = Integer.MAX_VALUE;
     static final int POSITION_BOTTOM = Integer.MIN_VALUE;
 
+
+    /**
+     * Counter for next free stack ID to use for dynamic activity stacks. Unique across displays.
+     */
+    private static int sNextFreeStackId = 0;
+
     private ActivityStackSupervisor mSupervisor;
     /** Actual Display this object tracks. */
     int mDisplayId;
@@ -231,6 +237,10 @@ class ActivityDisplay extends ConfigurationContainer<ActivityStack> {
         return getOrCreateStack(windowingMode, activityType, onTop);
     }
 
+    private int getNextStackId() {
+        return sNextFreeStackId++;
+    }
+
     /**
      * Creates a stack matching the input windowing mode and activity type on this display.
      * @param windowingMode The windowing mode the stack should be created in. If
@@ -278,7 +288,7 @@ class ActivityDisplay extends ConfigurationContainer<ActivityStack> {
             }
         }
 
-        final int stackId = mSupervisor.getNextStackId();
+        final int stackId = getNextStackId();
         return createStackUnchecked(windowingMode, activityType, stackId, onTop);
     }
 
diff --git a/services/core/java/com/android/server/am/ActivityStackSupervisor.java b/services/core/java/com/android/server/am/ActivityStackSupervisor.java
index 48c08a5718b9f..cd3b21c63ad64 100644
--- a/services/core/java/com/android/server/am/ActivityStackSupervisor.java
+++ b/services/core/java/com/android/server/am/ActivityStackSupervisor.java
@@ -298,9 +298,6 @@ public class ActivityStackSupervisor extends ConfigurationContainer implements D
 
     private LaunchingBoundsController mLaunchingBoundsController;
 
-    /** Counter for next free stack ID to use for dynamic activity stacks. */
-    private int mNextFreeStackId = 0;
-
     /**
      * Maps the task identifier that activities are currently being started in to the userId of the
      * task. Each time a new task is created, the entry for the userId of the task is incremented
@@ -2904,16 +2901,6 @@ public class ActivityStackSupervisor extends ConfigurationContainer implements D
         }
     }
 
-    int getNextStackId() {
-        while (true) {
-            if (getStack(mNextFreeStackId) == null) {
-                break;
-            }
-            mNextFreeStackId++;
-        }
-        return mNextFreeStackId;
-    }
-
     /**
      * Called to restore the state of the task into the stack that it's supposed to go into.
      *