Removed mStackId (86/n)

mStackId is the same as mTaskId since all stacks are now tasks
Also, switch some of the users to use getRootTask()/getRootTaskId()
where it makes sense.

Bug: 80414790
Test: Existing tests pass.
Change-Id: Ie625bf8734ca01646f6f45705edf0eb240819c24
diff --git a/services/core/java/com/android/server/wm/ActivityRecord.java b/services/core/java/com/android/server/wm/ActivityRecord.java
index 0e13e6c..fd514a4 100644
--- a/services/core/java/com/android/server/wm/ActivityRecord.java
+++ b/services/core/java/com/android/server/wm/ActivityRecord.java
@@ -31,7 +31,6 @@
 import static android.app.ActivityOptions.ANIM_THUMBNAIL_SCALE_DOWN;
 import static android.app.ActivityOptions.ANIM_THUMBNAIL_SCALE_UP;
 import static android.app.ActivityOptions.ANIM_UNDEFINED;
-import static android.app.ActivityTaskManager.INVALID_STACK_ID;
 import static android.app.ActivityTaskManager.INVALID_TASK_ID;
 import static android.app.AppOpsManager.MODE_ALLOWED;
 import static android.app.AppOpsManager.OP_PICTURE_IN_PICTURE;
@@ -1426,7 +1425,7 @@
                 return null;
             }
             ActivityRecord r = token.weakActivity.get();
-            if (r == null || r.getActivityStack() == null) {
+            if (r == null || r.getRootTask() == null) {
                 return null;
             }
             return r;
@@ -2055,19 +2054,17 @@
         }
     }
 
-    /**
-     * @return Stack value from current task, null if there is no task.
-     */
-    ActivityStack getActivityStack() {
-        return task != null ? task.getStack() : null;
+    /** @return Root task of this activity, null if there is no task. */
+    ActivityStack getRootTask() {
+        return task != null ? (ActivityStack) task.getRootTask() : null;
     }
 
-    int getStackId() {
-        return getActivityStack() != null ? getActivityStack().mStackId : INVALID_STACK_ID;
+    int getRootTaskId() {
+        return task != null ? task.getRootTaskId() : INVALID_TASK_ID;
     }
 
     DisplayContent getDisplay() {
-        final ActivityStack stack = getActivityStack();
+        final ActivityStack stack = getRootTask();
         return stack != null ? stack.getDisplay() : null;
     }
 
@@ -2088,7 +2085,7 @@
         mWmService.mWindowPlacerLocked.requestTraversal();
 
         if (changed && task != null && !occludesParent) {
-            getActivityStack().convertActivityToTranslucent(this);
+            getRootTask().convertActivityToTranslucent(this);
         }
         // Always ensure visibility if this activity doesn't occlude parent, so the
         // {@link #returningOptions} of the activity under this one can be applied in
@@ -2123,7 +2120,7 @@
     }
 
     boolean isInStackLocked() {
-        final ActivityStack stack = getActivityStack();
+        final ActivityStack stack = getRootTask();
         return stack != null && stack.isInStackLocked(this) != null;
     }
 
@@ -2325,7 +2322,7 @@
             return false;
         }
 
-        final ActivityStack stack = getActivityStack();
+        final ActivityStack stack = getRootTask();
         if (stack == null) {
             Slog.w(TAG, "moveActivityStackToFront: invalid task or stack: activity="
                     + this + " task=" + task);
@@ -2456,7 +2453,7 @@
             return FINISH_RESULT_CANCELLED;
         }
 
-        final ActivityStack stack = getActivityStack();
+        final ActivityStack stack = getRootTask();
         final boolean mayAdjustTop = (isState(RESUMED) || stack.mResumedActivity == null)
                 && stack.isFocusedStackOnDisplay();
         final boolean shouldAdjustGlobalFocus = mayAdjustTop
@@ -2497,7 +2494,7 @@
                     final DisplayContent display = stack.getDisplay();
                     next = display.topRunningActivity();
                     if (next != null) {
-                        display.positionStackAtTop(next.getActivityStack(),
+                        display.positionStackAtTop(next.getRootTask(),
                                 false /* includingParents */, "finish-display-top");
                     }
                 }
@@ -2666,7 +2663,7 @@
         mStackSupervisor.mStoppingActivities.remove(this);
         mStackSupervisor.mGoingToSleepActivities.remove(this);
 
-        final ActivityStack stack = getActivityStack();
+        final ActivityStack stack = getRootTask();
         final DisplayContent display = getDisplay();
         // TODO(b/137329632): Exclude current activity when looking for the next one with
         // DisplayContent#topRunningActivity().
@@ -2824,7 +2821,7 @@
     boolean safelyDestroy(String reason) {
         if (isDestroyable()) {
             if (DEBUG_SWITCH) {
-                final ActivityStack stack = getActivityStack();
+                final ActivityStack stack = getRootTask();
                 Slog.v(TAG_SWITCH, "Safely destroying " + this + " in state " + getState()
                         + " resumed=" + stack.mResumedActivity
                         + " pausing=" + stack.mPausingActivity
@@ -3612,7 +3609,7 @@
     }
 
     final boolean isSleeping() {
-        final ActivityStack stack = getActivityStack();
+        final ActivityStack stack = getRootTask();
         return stack != null ? stack.shouldSleepActivities() : mAtmService.isSleepingLocked();
     }
 
@@ -4449,7 +4446,7 @@
             return visibleIgnoringKeyguard;
         }
 
-        final ActivityStack stack = getActivityStack();
+        final ActivityStack stack = getRootTask();
         if (stack == null) {
             return false;
         }
@@ -4471,7 +4468,7 @@
     }
 
     boolean shouldBeVisible() {
-        final ActivityStack stack = getActivityStack();
+        final ActivityStack stack = getRootTask();
         if (stack == null) {
             return false;
         }
@@ -4492,7 +4489,7 @@
         // If this activity is paused, tell it to now show its window.
         if (DEBUG_VISIBILITY) Slog.v(TAG_VISIBILITY,
                 "Making visible and scheduling visibility: " + this);
-        final ActivityStack stack = getActivityStack();
+        final ActivityStack stack = getRootTask();
         try {
             if (stack.mTranslucentActivityWaiting != null) {
                 updateOptionsLocked(returningOptions);
@@ -4578,7 +4575,7 @@
             if (DEBUG_VISIBILITY) {
                 Slog.v(TAG_VISIBILITY, "Resume visible activity, " + this);
             }
-            return getActivityStack().resumeTopActivityUncheckedLocked(activeActivity /* prev */,
+            return getRootTask().resumeTopActivityUncheckedLocked(activeActivity /* prev */,
                     null /* options */);
         } else if (shouldPauseActivity(activeActivity)) {
             if (DEBUG_VISIBILITY) {
@@ -4643,7 +4640,7 @@
      */
     private boolean shouldBeResumed(ActivityRecord activeActivity) {
         return shouldMakeActive(activeActivity) && isFocusable()
-                && getActivityStack().getVisibility(activeActivity) == STACK_VISIBILITY_VISIBLE
+                && getRootTask().getVisibility(activeActivity) == STACK_VISIBILITY_VISIBLE
                 && canResumeByCompat();
     }
 
@@ -4672,7 +4669,7 @@
         // ActivityStack#ensureActivitiesVisible will bring the activity to a proper
         // active state.
         if (!isState(STARTED, RESUMED, PAUSED, STOPPED, STOPPING)
-                || getActivityStack().mTranslucentActivityWaiting != null) {
+                || getRootTask().mTranslucentActivityWaiting != null) {
             return false;
         }
 
@@ -4769,7 +4766,7 @@
         mStackSupervisor.reportResumedActivityLocked(this);
 
         resumeKeyDispatchingLocked();
-        final ActivityStack stack = getActivityStack();
+        final ActivityStack stack = getRootTask();
         mStackSupervisor.mNoAnimActivities.clear();
 
         // Mark the point when the activity is resuming
@@ -4862,7 +4859,7 @@
 
     void stopIfPossible() {
         if (DEBUG_SWITCH) Slog.d(TAG_SWITCH, "Stopping: " + this);
-        final ActivityStack stack = getActivityStack();
+        final ActivityStack stack = getRootTask();
         if (isNoHistory()) {
             if (!finishing) {
                 if (!stack.shouldSleepActivities()) {
@@ -4919,7 +4916,7 @@
 
     void activityStopped(Bundle newIcicle, PersistableBundle newPersistentState,
             CharSequence description) {
-        final ActivityStack stack = getActivityStack();
+        final ActivityStack stack = getRootTask();
         final boolean isStopping = mState == STOPPING;
         if (!isStopping && mState != RESTARTING_PROCESS) {
             Slog.i(TAG, "Activity reported stop, but no longer stopping: " + this);
@@ -4969,7 +4966,7 @@
             mStackSupervisor.mStoppingActivities.add(this);
         }
 
-        final ActivityStack stack = getActivityStack();
+        final ActivityStack stack = getRootTask();
         // If we already have a few activities waiting to stop, then give up on things going idle
         // and start clearing them out. Or if r is the last of activity of the last task the stack
         // will be empty and must be cleared immediately.
@@ -5005,7 +5002,7 @@
             return false;
         }
 
-        final ActivityStack stack = getActivityStack();
+        final ActivityStack stack = getRootTask();
         if (stack == null) {
             return false;
         }
@@ -5021,7 +5018,7 @@
 
     void finishLaunchTickingLocked() {
         launchTickTime = 0;
-        final ActivityStack stack = getActivityStack();
+        final ActivityStack stack = getRootTask();
         if (stack == null) {
             return;
         }
@@ -5491,13 +5488,13 @@
 
     static ActivityRecord isInStackLocked(IBinder token) {
         final ActivityRecord r = ActivityRecord.forTokenLocked(token);
-        return (r != null) ? r.getActivityStack().isInStackLocked(r) : null;
+        return (r != null) ? r.getRootTask().isInStackLocked(r) : null;
     }
 
     static ActivityStack getStackLocked(IBinder token) {
         final ActivityRecord r = ActivityRecord.isInStackLocked(token);
         if (r != null) {
-            return r.getActivityStack();
+            return r.getRootTask();
         }
         return null;
     }
@@ -5507,7 +5504,7 @@
      *         {@link android.view.Display#INVALID_DISPLAY} if not attached.
      */
     int getDisplayId() {
-        final ActivityStack stack = getActivityStack();
+        final ActivityStack stack = getRootTask();
         if (stack == null) {
             return INVALID_DISPLAY;
         }
@@ -5519,7 +5516,7 @@
             // This would be redundant.
             return false;
         }
-        final ActivityStack stack = getActivityStack();
+        final ActivityStack stack = getRootTask();
         if (isState(RESUMED) || stack == null || this == stack.mPausingActivity || !mHaveState
                 || !stopped) {
             // We're not ready for this kind of thing.
@@ -6755,7 +6752,7 @@
     private void applyAspectRatio(Rect outBounds, Rect containingAppBounds,
             Rect containingBounds) {
         final float maxAspectRatio = info.maxAspectRatio;
-        final ActivityStack stack = getActivityStack();
+        final ActivityStack stack = getRootTask();
         final float minAspectRatio = info.minAspectRatio;
 
         if (task == null || stack == null || (inMultiWindowMode() && !shouldUseSizeCompatMode())
@@ -6861,7 +6858,7 @@
      */
     boolean ensureActivityConfiguration(int globalChanges, boolean preserveWindow,
             boolean ignoreVisibility) {
-        final ActivityStack stack = getActivityStack();
+        final ActivityStack stack = getRootTask();
         if (stack.mConfigWillChange) {
             if (DEBUG_SWITCH || DEBUG_CONFIGURATION) Slog.v(TAG_CONFIGURATION,
                     "Skipping config check (will change): " + this);
@@ -7406,7 +7403,7 @@
         if (!getTurnScreenOnFlag()) {
             return false;
         }
-        final ActivityStack stack = getActivityStack();
+        final ActivityStack stack = getRootTask();
         return stack != null &&
                 stack.checkKeyguardVisibility(this, true /* shouldBeVisible */, true /* isTop */);
     }
diff --git a/services/core/java/com/android/server/wm/ActivityStack.java b/services/core/java/com/android/server/wm/ActivityStack.java
index 60e0f51e..78e681f 100644
--- a/services/core/java/com/android/server/wm/ActivityStack.java
+++ b/services/core/java/com/android/server/wm/ActivityStack.java
@@ -277,9 +277,6 @@
 
     int mCurrentUser;
 
-    /** Unique identifier */
-    final int mStackId;
-
     /** For comparison with DisplayContent bounds. */
     private Rect mTmpRect = new Rect();
     private Rect mTmpRect2 = new Rect();
@@ -633,7 +630,6 @@
                 _realActivitySuspended, userSetupComplete, minWidth, minHeight, info, _voiceSession,
                 _voiceInteractor, stack);
 
-        mStackId = mTaskId;
         mDockedStackMinimizeThickness = mWmService.mContext.getResources().getDimensionPixelSize(
                 com.android.internal.R.dimen.docked_stack_minimize_thickness);
         EventLogTags.writeWmStackCreated(id);
@@ -1118,7 +1114,7 @@
             return null;
         }
         final Task task = r.getTask();
-        final ActivityStack stack = r.getActivityStack();
+        final ActivityStack stack = r.getRootTask();
         if (stack != null && task.mChildren.contains(r) && mChildren.contains(task)) {
             if (stack != this) Slog.w(TAG,
                     "Illegal state! task does not point to stack it is in.");
@@ -1347,7 +1343,7 @@
 
     private boolean containsActivityFromStack(List<ActivityRecord> rs) {
         for (ActivityRecord r : rs) {
-            if (r.getActivityStack() == this) {
+            if (r.getRootTask() == this) {
                 return true;
             }
         }
@@ -1873,7 +1869,7 @@
         final DisplayContent displayContent = getDisplay();
         if (displayContent == null) {
             throw new IllegalStateException("Stack is not attached to any display, stackId="
-                    + mStackId);
+                    + getRootTaskId());
         }
 
         final int flags = displayContent.mDisplay.getFlags();
@@ -2592,7 +2588,7 @@
             return false;
         }
         final ActivityStack targetStack = toFrontTask != null
-                ? toFrontTask.getStack() : toFrontActivity.getActivityStack();
+                ? toFrontTask.getStack() : toFrontActivity.getRootTask();
         if (targetStack != null && targetStack.isActivityTypeAssistant()) {
             // Ensure the task/activity being brought forward is not the assistant
             return false;
@@ -3073,7 +3069,7 @@
         }
 
         ActivityRecord topActivity = getDisplay().topRunningActivity();
-        ActivityStack topStack = topActivity.getActivityStack();
+        ActivityStack topStack = topActivity.getRootTask();
         if (topStack != null && topStack != this && topActivity.isState(RESUMED)) {
             // The new top activity is already resumed, so there's a good chance that nothing will
             // get resumed below. So, update visibility now in case the transition is closed
@@ -3102,7 +3098,7 @@
             return;
         }
 
-        Trace.traceBegin(TRACE_TAG_WINDOW_MANAGER, "stack.resize_" + mStackId);
+        Trace.traceBegin(TRACE_TAG_WINDOW_MANAGER, "stack.resize_" + getRootTaskId());
         mAtmService.deferWindowLayout();
         try {
             // Update override configurations of all tasks in the stack.
@@ -3219,7 +3215,7 @@
 
     boolean dump(FileDescriptor fd, PrintWriter pw, boolean dumpAll, boolean dumpClient,
             String dumpPackage, boolean needSep) {
-        pw.println("  Stack #" + mStackId
+        pw.println("  Stack #" + getRootTaskId()
                 + ": type=" + activityTypeToString(getActivityType())
                 + " mode=" + windowingModeToString(getWindowingMode()));
         pw.println("  isSleeping=" + shouldSleepActivities());
@@ -4484,7 +4480,7 @@
 
     @Override
     void dump(PrintWriter pw, String prefix, boolean dumpAll) {
-        pw.println(prefix + "mStackId=" + mStackId);
+        pw.println(prefix + "mStackId=" + getRootTaskId());
         pw.println(prefix + "mDeferRemoval=" + mDeferRemoval);
         pw.println(prefix + "mBounds=" + getRawBounds().toShortString());
         if (mMinimizeAmount != 0f) {
@@ -4515,14 +4511,6 @@
         mAnimatingActivityRegistry.dump(pw, "AnimatingApps:", prefix);
     }
 
-    String getName() {
-        return toShortString();
-    }
-
-    public String toShortString() {
-        return "Stack=" + mStackId;
-    }
-
     /**
      * For docked workspace (or workspace that's side-by-side to the docked), provides
      * information which side of the screen was the dock anchored.
@@ -4863,7 +4851,7 @@
             @WindowTraceLogLevel int logLevel) {
         final long token = proto.start(fieldId);
         dumpDebugInnerStackOnly(proto, STACK, logLevel);
-        proto.write(com.android.server.am.ActivityStackProto.ID, mStackId);
+        proto.write(com.android.server.am.ActivityStackProto.ID, getRootTaskId());
 
         forAllTasks((t) -> {
             t.dumpDebugInner(proto, com.android.server.am.ActivityStackProto.TASKS, logLevel);
@@ -4891,7 +4879,7 @@
 
         final long token = proto.start(fieldId);
         super.dumpDebug(proto, WINDOW_CONTAINER, logLevel);
-        proto.write(StackProto.ID, mStackId);
+        proto.write(StackProto.ID, getRootTaskId());
         forAllTasks((t) -> {
             t.dumpDebugInnerTaskOnly(proto, StackProto.TASKS, logLevel);
         }, true /* traverseTopToBottom */, this);
diff --git a/services/core/java/com/android/server/wm/ActivityStackSupervisor.java b/services/core/java/com/android/server/wm/ActivityStackSupervisor.java
index f2ce7e8..120ae86 100644
--- a/services/core/java/com/android/server/wm/ActivityStackSupervisor.java
+++ b/services/core/java/com/android/server/wm/ActivityStackSupervisor.java
@@ -824,7 +824,7 @@
                         false /* markFrozenIfConfigChanged */, true /* deferResume */);
             }
 
-            if (r.getActivityStack().checkKeyguardVisibility(r, true /* shouldBeVisible */,
+            if (r.getRootTask().checkKeyguardVisibility(r, true /* shouldBeVisible */,
                     true /* isTop */) && r.allowMoveToFront()) {
                 // We only set the visibility to true if the activity is not being launched in
                 // background, and is allowed to be visible based on keyguard state. This avoids
@@ -1891,13 +1891,13 @@
     // TODO: Look into changing users to this method to DisplayContent.resolveWindowingMode()
     ActivityStack getReparentTargetStack(Task task, ActivityStack stack, boolean toTop) {
         final ActivityStack prevStack = task.getStack();
-        final int stackId = stack.mStackId;
+        final int rootTaskId = stack.mTaskId;
         final boolean inMultiWindowMode = stack.inMultiWindowMode();
 
         // Check that we aren't reparenting to the same stack that the task is already in
-        if (prevStack != null && prevStack.mStackId == stackId) {
+        if (prevStack != null && prevStack.mTaskId == rootTaskId) {
             Slog.w(TAG, "Can not reparent to same stack, task=" + task
-                    + " already in stackId=" + stackId);
+                    + " already in stackId=" + rootTaskId);
             return prevStack;
         }
 
@@ -1912,7 +1912,7 @@
         // multi-display.
         if (stack.getDisplayId() != DEFAULT_DISPLAY && !mService.mSupportsMultiDisplay) {
             throw new IllegalArgumentException("Device doesn't support multi-display, can not"
-                    + " reparent task=" + task + " to stackId=" + stackId);
+                    + " reparent task=" + task + " to stackId=" + rootTaskId);
         }
 
         // Ensure that we aren't trying to move into a freeform stack without freeform support
@@ -1993,7 +1993,7 @@
 
     void activitySleptLocked(ActivityRecord r) {
         mGoingToSleepActivities.remove(r);
-        final ActivityStack s = r.getActivityStack();
+        final ActivityStack s = r.getRootTask();
         if (s != null) {
             s.checkReadyForSleep();
         } else {
@@ -2029,7 +2029,7 @@
         // A resumed activity cannot be stopping. remove from list
         mStoppingActivities.remove(r);
 
-        final ActivityStack stack = r.getActivityStack();
+        final ActivityStack stack = r.getRootTask();
         if (stack.getDisplay().allResumedActivitiesComplete()) {
             mRootWindowContainer.ensureActivitiesVisible(null, 0, !PRESERVE_WINDOWS);
             // Make sure activity & window visibility should be identical
@@ -2082,7 +2082,7 @@
             if (DEBUG_STATES) Slog.v(TAG, "Stopping " + s + ": nowVisible=" + s.nowVisible
                     + " animating=" + animating + " finishing=" + s.finishing);
 
-            final ActivityStack stack = s.getActivityStack();
+            final ActivityStack stack = s.getRootTask();
             final boolean shouldSleepOrShutDown = stack != null
                     ? stack.shouldSleepOrShutDownActivities()
                     : mService.isSleepingOrShuttingDownLocked();
@@ -2473,7 +2473,7 @@
         final ActivityRecord r = ActivityRecord.isInStackLocked(token);
         if (r != null) {
             r.finishRelaunching();
-            if (r.getActivityStack().shouldSleepOrShutDownActivities()) {
+            if (r.getRootTask().shouldSleepOrShutDownActivities()) {
                 r.setSleeping(true, true);
             }
         }
diff --git a/services/core/java/com/android/server/wm/ActivityStarter.java b/services/core/java/com/android/server/wm/ActivityStarter.java
index 6587226..457a305 100644
--- a/services/core/java/com/android/server/wm/ActivityStarter.java
+++ b/services/core/java/com/android/server/wm/ActivityStarter.java
@@ -937,7 +937,7 @@
         }
 
         final ActivityStack resultStack = resultRecord == null
-                ? null : resultRecord.getActivityStack();
+                ? null : resultRecord.getRootTask();
 
         if (err != START_SUCCESS) {
             if (resultRecord != null) {
@@ -1425,7 +1425,7 @@
      * @return the stack where the successful started activity resides.
      */
     private @Nullable ActivityStack handleStartResult(@NonNull ActivityRecord started, int result) {
-        final ActivityStack currentStack = started.getActivityStack();
+        final ActivityStack currentStack = started.getRootTask();
         ActivityStack startedActivityStack = currentStack != null ? currentStack : mTargetStack;
 
         if (ActivityManager.isStartResultSuccessful(result)) {
@@ -1446,7 +1446,7 @@
         // If we are not able to proceed, disassociate the activity from the task. Leaving an
         // activity in an incomplete state can lead to issues, such as performing operations
         // without a window container.
-        final ActivityStack stack = mStartActivity.getActivityStack();
+        final ActivityStack stack = mStartActivity.getRootTask();
         if (stack != null) {
             mStartActivity.finishIfPossible("startActivity", true /* oomAdj */);
         }
@@ -2231,7 +2231,7 @@
             return;
         }
         if (!mSourceRecord.finishing) {
-            mSourceStack = mSourceRecord.getActivityStack();
+            mSourceStack = mSourceRecord.getRootTask();
             return;
         }
 
@@ -2314,7 +2314,7 @@
      * @return {@link ActivityRecord} brought to front.
      */
     private void setTargetStackIfNeeded(ActivityRecord intentActivity) {
-        mTargetStack = intentActivity.getActivityStack();
+        mTargetStack = intentActivity.getRootTask();
         mTargetStack.mLastPausedActivity = null;
         // If the target task is not in the front, then we need to bring it to the front...
         // except...  well, with SINGLE_TASK_LAUNCH it's not entirely clear. We'd like to have
@@ -2371,7 +2371,7 @@
         }
         // Need to update mTargetStack because if task was moved out of it, the original stack may
         // be destroyed.
-        mTargetStack = intentActivity.getActivityStack();
+        mTargetStack = intentActivity.getRootTask();
         mSupervisor.handleNonResizableTaskIfNeeded(intentActivity.getTask(),
                 WINDOWING_MODE_UNDEFINED, DEFAULT_DISPLAY, mTargetStack);
     }
@@ -2422,10 +2422,11 @@
             return;
         }
 
-        final ActivityStack stack = task.getStack();
-        if (stack != null && stack.inPinnedWindowingMode()) {
-            mService.animateResizePinnedStack(stack.mStackId, bounds, -1);
+        final Task rootTask = task.getRootTask();
+        if (rootTask != null && rootTask.inPinnedWindowingMode()) {
+            mService.animateResizePinnedStack(rootTask.mTaskId, bounds, -1);
         } else {
+            // TODO: I don't believe it is possible to reach this else condition anymore...
             task.setBounds(bounds);
         }
     }
diff --git a/services/core/java/com/android/server/wm/ActivityTaskManagerService.java b/services/core/java/com/android/server/wm/ActivityTaskManagerService.java
index 8491bc2..9f4c581 100644
--- a/services/core/java/com/android/server/wm/ActivityTaskManagerService.java
+++ b/services/core/java/com/android/server/wm/ActivityTaskManagerService.java
@@ -1612,7 +1612,7 @@
             if (mController != null) {
                 // Find the first activity that is not finishing.
                 final ActivityRecord next =
-                        r.getActivityStack().topRunningActivity(token, INVALID_TASK_ID);
+                        r.getRootTask().topRunningActivity(token, INVALID_TASK_ID);
                 if (next != null) {
                     // ask watcher if this is allowed
                     boolean resumeOK = true;
@@ -2037,7 +2037,7 @@
         synchronized (mGlobalLock) {
             ActivityRecord r = mRootWindowContainer.isInAnyStack(token);
             if (r != null) {
-                r.getActivityStack().notifyActivityDrawnLocked(r);
+                r.getRootTask().notifyActivityDrawnLocked(r);
             }
         }
     }
@@ -2073,7 +2073,7 @@
             synchronized (mGlobalLock) {
                 ActivityStack focusedStack = getTopDisplayFocusedStack();
                 if (focusedStack != null) {
-                    return mRootWindowContainer.getStackInfo(focusedStack.mStackId);
+                    return mRootWindowContainer.getStackInfo(focusedStack.mTaskId);
                 }
                 return null;
             }
@@ -2175,7 +2175,7 @@
         synchronized (mGlobalLock) {
             final ActivityRecord srec = ActivityRecord.forTokenLocked(token);
             if (srec != null) {
-                return srec.getActivityStack().shouldUpRecreateTaskLocked(srec, destAffinity);
+                return srec.getRootTask().shouldUpRecreateTaskLocked(srec, destAffinity);
             }
         }
         return false;
@@ -2188,7 +2188,7 @@
         synchronized (mGlobalLock) {
             final ActivityRecord r = ActivityRecord.forTokenLocked(token);
             if (r != null) {
-                return r.getActivityStack().navigateUpTo(
+                return r.getRootTask().navigateUpTo(
                         r, destIntent, resultCode, resultData);
             }
             return false;
@@ -2354,7 +2354,7 @@
             if (r == null) {
                 return;
             }
-            ActivityStack stack = r.getActivityStack();
+            ActivityStack stack = r.getRootTask();
             if (stack != null && stack.isSingleTaskInstance()) {
                 // Single-task stacks are used for activities which are presented in floating
                 // windows above full screen activities. Instead of directly finishing the
@@ -2582,7 +2582,7 @@
                         r, resultWho, requestCode);
                 // TODO: This should probably only loop over the task since you need to be in the
                 // same task to return results.
-                r.getActivityStack().forAllActivities(c);
+                r.getRootTask().forAllActivities(c);
                 c.recycle();
 
                 updateOomAdj();
@@ -3215,7 +3215,7 @@
                                     + ainfo.applicationInfo.uid + ", calling uid=" + callingUid);
                 }
 
-                final ActivityStack stack = r.getActivityStack();
+                final ActivityStack stack = r.getRootTask();
                 final Task task = stack.createTask(
                         mStackSupervisor.getNextTaskIdForUser(r.mUserId), ainfo, intent, !ON_TOP);
                 if (!mRecentTasks.addToBottom(task)) {
@@ -3551,7 +3551,7 @@
                                     + token);
                 }
 
-                final ActivityStack stack = r.getActivityStack();
+                final ActivityStack stack = r.getRootTask();
                 if (stack == null) {
                     throw new IllegalStateException("toggleFreeformWindowingMode: the activity "
                             + "doesn't have a stack");
@@ -4235,14 +4235,14 @@
     }
 
     private boolean isInPictureInPictureMode(ActivityRecord r) {
-        if (r == null || r.getActivityStack() == null || !r.inPinnedWindowingMode()
-                || r.getActivityStack().isInStackLocked(r) == null) {
+        if (r == null || r.getRootTask() == null || !r.inPinnedWindowingMode()
+                || r.getRootTask().isInStackLocked(r) == null) {
             return false;
         }
 
         // If we are animating to fullscreen then we have already dispatched the PIP mode
         // changed, so we should reflect that check here as well.
-        final ActivityStack taskStack = r.getActivityStack();
+        final ActivityStack taskStack = r.getRootTask();
         return !taskStack.isAnimatingBoundsToFullscreen();
     }
 
@@ -4281,7 +4281,7 @@
                                 r.pictureInPictureArgs.getSourceRectHint());
                         mRootWindowContainer.moveActivityToPinnedStack(
                                 r, sourceBounds, aspectRatio, "enterPictureInPictureMode");
-                        final ActivityStack stack = r.getActivityStack();
+                        final ActivityStack stack = r.getRootTask();
                         stack.setPictureInPictureAspectRatio(aspectRatio);
                         stack.setPictureInPictureActions(actions);
                         MetricsLoggerWrapper.logPictureInPictureEnter(mContext,
@@ -4326,7 +4326,7 @@
                     // If the activity is already in picture-in-picture, update the pinned stack now
                     // if it is not already expanding to fullscreen. Otherwise, the arguments will
                     // be used the next time the activity enters PiP
-                    final ActivityStack stack = r.getActivityStack();
+                    final ActivityStack stack = r.getRootTask();
                     if (!stack.isAnimatingBoundsToFullscreen()) {
                         stack.setPictureInPictureAspectRatio(
                                 r.pictureInPictureArgs.getAspectRatio());
@@ -4923,7 +4923,7 @@
             Slog.i(TAG, "Moving " + r.shortComponentName + " from display " + r.getDisplayId()
                     + " to main display for VR");
             mRootWindowContainer.moveStackToDisplay(
-                    r.getStackId(), DEFAULT_DISPLAY, true /* toTop */);
+                    r.getRootTaskId(), DEFAULT_DISPLAY, true /* toTop */);
         }
         mH.post(() -> {
             if (!mVrController.onVrModeChanged(r)) {
@@ -6752,7 +6752,7 @@
                 int requestCode, int resultCode, Intent data) {
             synchronized (mGlobalLock) {
                 final ActivityRecord r = ActivityRecord.isInStackLocked(activityToken);
-                if (r != null && r.getActivityStack() != null) {
+                if (r != null && r.getRootTask() != null) {
                     r.sendResult(callingUid, resultWho, requestCode, resultCode, data);
                 }
             }
diff --git a/services/core/java/com/android/server/wm/DisplayContent.java b/services/core/java/com/android/server/wm/DisplayContent.java
index 707c773..ed782c2 100644
--- a/services/core/java/com/android/server/wm/DisplayContent.java
+++ b/services/core/java/com/android/server/wm/DisplayContent.java
@@ -2772,7 +2772,7 @@
         proto.write(SINGLE_TASK_INSTANCE, mSingleTaskInstance);
         final ActivityStack focusedStack = getFocusedStack();
         if (focusedStack != null) {
-            proto.write(FOCUSED_STACK_ID, focusedStack.mStackId);
+            proto.write(FOCUSED_STACK_ID, focusedStack.getRootTaskId());
             final ActivityRecord focusedActivity = focusedStack.getDisplay().getResumedActivity();
             if (focusedActivity != null) {
                 focusedActivity.writeIdentifierToProto(proto, RESUMED_ACTIVITY);
@@ -5655,8 +5655,8 @@
             if (currentFocusedStack != prevFocusedStack) {
                 mLastFocusedStack = prevFocusedStack;
                 EventLogTags.writeWmFocusedStack(mRootWindowContainer.mCurrentUser, mDisplayId,
-                        currentFocusedStack == null ? -1 : currentFocusedStack.getStackId(),
-                        mLastFocusedStack == null ? -1 : mLastFocusedStack.getStackId(),
+                        currentFocusedStack == null ? -1 : currentFocusedStack.getRootTaskId(),
+                        mLastFocusedStack == null ? -1 : mLastFocusedStack.getRootTaskId(),
                         updateLastFocusedStackReason);
             }
         }
@@ -5664,10 +5664,10 @@
         onStackOrderChanged(stack);
     }
 
-    ActivityStack getStack(int stackId) {
+    ActivityStack getStack(int rootTaskId) {
         for (int i = getStackCount() - 1; i >= 0; --i) {
             final ActivityStack stack = getStackAt(i);
-            if (stack.mStackId == stackId) {
+            if (stack.getRootTaskId() == rootTaskId) {
                 return stack;
             }
         }
@@ -6704,7 +6704,7 @@
 
     public void dumpStacks(PrintWriter pw) {
         for (int i = getStackCount() - 1; i >= 0; --i) {
-            pw.print(getStackAt(i).mStackId);
+            pw.print(getStackAt(i).getRootTaskId());
             if (i > 0) {
                 pw.print(",");
             }
diff --git a/services/core/java/com/android/server/wm/LaunchParamsController.java b/services/core/java/com/android/server/wm/LaunchParamsController.java
index 03e1322..a9820ef 100644
--- a/services/core/java/com/android/server/wm/LaunchParamsController.java
+++ b/services/core/java/com/android/server/wm/LaunchParamsController.java
@@ -138,7 +138,7 @@
         try {
             if (mTmpParams.hasPreferredDisplay()
                     && mTmpParams.mPreferredDisplayId != task.getStack().getDisplay().mDisplayId) {
-                mService.moveStackToDisplay(task.getStackId(), mTmpParams.mPreferredDisplayId);
+                mService.moveStackToDisplay(task.getRootTaskId(), mTmpParams.mPreferredDisplayId);
             }
 
             if (mTmpParams.hasWindowingMode()
diff --git a/services/core/java/com/android/server/wm/RootWindowContainer.java b/services/core/java/com/android/server/wm/RootWindowContainer.java
index 8202833..3eddd85 100644
--- a/services/core/java/com/android/server/wm/RootWindowContainer.java
+++ b/services/core/java/com/android/server/wm/RootWindowContainer.java
@@ -1656,7 +1656,7 @@
         // Only resume home activity if isn't finishing.
         if (r != null && !r.finishing) {
             r.moveFocusableActivityToTop(myReason);
-            return resumeFocusedStacksTopActivities(r.getActivityStack(), prev, null);
+            return resumeFocusedStacksTopActivities(r.getRootTask(), prev, null);
         }
         return startHomeOnDisplay(mCurrentUser, myReason, displayId);
     }
@@ -1980,7 +1980,7 @@
     }
 
     boolean switchUser(int userId, UserState uss) {
-        final int focusStackId = getTopDisplayFocusedStack().getStackId();
+        final int focusStackId = getTopDisplayFocusedStack().getRootTaskId();
         // We dismiss the docked stack whenever we switch users.
         final ActivityStack dockedStack = getDefaultDisplay().getSplitScreenPrimaryStack();
         if (dockedStack != null) {
@@ -1994,7 +1994,7 @@
 
         mUserStackInFront.put(mCurrentUser, focusStackId);
         final int restoreStackId =
-                mUserStackInFront.get(userId, getDefaultDisplay().getHomeStack().mStackId);
+                mUserStackInFront.get(userId, getDefaultDisplay().getHomeStack().getRootTaskId());
         mCurrentUser = userId;
 
         mStackSupervisor.mStartingUsers.add(uss);
@@ -2034,8 +2034,8 @@
      */
     void updateUserStack(int userId, ActivityStack stack) {
         if (userId != mCurrentUser) {
-            mUserStackInFront.put(userId, stack != null ? stack.getStackId()
-                    : getDefaultDisplay().getHomeStack().mStackId);
+            mUserStackInFront.put(userId, stack != null ? stack.getRootTaskId()
+                    : getDefaultDisplay().getHomeStack().getRootTaskId());
         }
     }
 
@@ -2108,7 +2108,7 @@
             String reason) {
         mService.deferWindowLayout();
 
-        final DisplayContent display = r.getActivityStack().getDisplay();
+        final DisplayContent display = r.getRootTask().getDisplay();
 
         try {
             final Task task = r.getTask();
@@ -2124,13 +2124,13 @@
 
             final ActivityStack stack;
             if (singleActivity) {
-                stack = r.getActivityStack();
+                stack = r.getRootTask();
             } else {
                 // In the case of multiple activities, we will create a new stack for it and then
                 // move the PIP activity into the stack.
                 // We will then perform a windowing mode change for both scenarios.
                 stack = display.createStack(
-                        r.getActivityStack().getRequestedOverrideWindowingMode(),
+                        r.getRootTask().getRequestedOverrideWindowingMode(),
                         r.getActivityType(), ON_TOP, r.info, r.intent);
                 // There are multiple activities in the task and moving the top activity should
                 // reveal/leave the other activities in their original task.
@@ -2398,7 +2398,7 @@
         ActivityManager.StackInfo info = new ActivityManager.StackInfo();
         stack.getBounds(info.bounds);
         info.displayId = display.mDisplayId;
-        info.stackId = stack.mStackId;
+        info.stackId = stack.mTaskId;
         info.stackToken = stack.mRemoteToken;
         info.userId = stack.mCurrentUser;
         info.visible = stack.shouldBeVisible(null);
@@ -2853,7 +2853,7 @@
             stack = candidateTask.getStack();
         }
         if (stack == null && r != null) {
-            stack = r.getActivityStack();
+            stack = r.getRootTask();
         }
         if (stack != null) {
             display = stack.getDisplay();
@@ -3244,7 +3244,7 @@
             if (resumedActivity == null || !resumedActivity.idle) {
                 if (DEBUG_STATES) {
                     Slog.d(TAG_STATES, "allResumedActivitiesIdle: stack="
-                            + stack.mStackId + " " + resumedActivity + " not idle");
+                            + stack.getRootTaskId() + " " + resumedActivity + " not idle");
                 }
                 return false;
             }
@@ -3599,7 +3599,7 @@
         // TODO(b/111541062): Update tests to look for resumed activities on all displays
         final ActivityStack focusedStack = getTopDisplayFocusedStack();
         if (focusedStack != null) {
-            proto.write(FOCUSED_STACK_ID, focusedStack.mStackId);
+            proto.write(FOCUSED_STACK_ID, focusedStack.getRootTaskId());
             final ActivityRecord focusedActivity = focusedStack.getDisplay().getResumedActivity();
             if (focusedActivity != null) {
                 focusedActivity.writeIdentifierToProto(proto, RESUMED_ACTIVITY);
diff --git a/services/core/java/com/android/server/wm/Task.java b/services/core/java/com/android/server/wm/Task.java
index 9e6cb68..c52966b 100644
--- a/services/core/java/com/android/server/wm/Task.java
+++ b/services/core/java/com/android/server/wm/Task.java
@@ -526,7 +526,7 @@
             ActivityStack stack) {
         super(atmService.mWindowManager);
 
-        EventLogTags.writeWmTaskCreated(_taskId, stack != null ? stack.mStackId : INVALID_STACK_ID);
+        EventLogTags.writeWmTaskCreated(_taskId, stack != null ? getRootTaskId() : INVALID_TASK_ID);
         mAtmService = atmService;
         mStackSupervisor = atmService.mStackSupervisor;
         mRootWindowContainer = mAtmService.mRootWindowContainer;
@@ -1080,7 +1080,7 @@
         // the display, so we should probably consolidate it there instead.
 
         if (getParent() == null && mDisplayContent != null) {
-            EventLogTags.writeWmStackRemoved(getStackId());
+            EventLogTags.writeWmStackRemoved(getRootTaskId());
             mDisplayContent = null;
             mWmService.mWindowPlacerLocked.requestTraversal();
         }
@@ -2344,17 +2344,27 @@
         return dc != null ? dc.mDisplayId : INVALID_DISPLAY;
     }
 
+    // TODO: Migrate callers to getRootTask()
     ActivityStack getStack() {
-        final WindowContainer parent = getParent();
-        return (ActivityStack) (parent instanceof ActivityStack ? parent : this);
+        return (ActivityStack) getRootTask();
     }
 
-    /**
-     * @return Id of current stack, {@link ActivityTaskManager#INVALID_STACK_ID} if no stack is set.
-     */
-    int getStackId() {
-        final ActivityStack stack = getStack();
-        return stack != null ? stack.mStackId : INVALID_STACK_ID;
+    /** @return Id of root task. */
+    int getRootTaskId() {
+        return getRootTask().mTaskId;
+    }
+
+    Task getRootTask() {
+        final WindowContainer parent = getParent();
+        if (parent == null) return this;
+
+        final Task parentTask = parent.asTask();
+        return parentTask == null ? this : parentTask.getRootTask();
+    }
+
+    // TODO(task-merge): Figure out what's the right thing to do for places that used it.
+    boolean isRootTask() {
+        return getRootTask() == this;
     }
 
     int getDescendantTaskCount() {
@@ -2736,7 +2746,7 @@
             // No need to check if the mode is allowed if it's leaving dragResize
             if (dragResizing && !DragResizeMode.isModeAllowedForStack(getStack(), dragResizeMode)) {
                 throw new IllegalArgumentException("Drag resize mode not allow for stack stackId="
-                        + getStack().mStackId + " dragResizeMode=" + dragResizeMode);
+                        + getRootTaskId() + " dragResizeMode=" + dragResizeMode);
             }
             mDragResizing = dragResizing;
             mDragResizeMode = dragResizeMode;
@@ -3010,11 +3020,6 @@
         return mTaskDescription;
     }
 
-    // TODO(task-merge): Figure out what's the right thing to do for places that used it.
-    boolean isRootTask() {
-        return getParent() == null || getParent().asTask() == null;
-    }
-
     @Override
     boolean fillsParent() {
         return matchParentBounds();
@@ -3167,7 +3172,7 @@
     void fillTaskInfo(TaskInfo info) {
         getNumRunningActivities(mReuseActivitiesReport);
         info.userId = mUserId;
-        info.stackId = getStackId();
+        info.stackId = getRootTaskId();
         info.taskId = mTaskId;
         info.displayId = getDisplayId();
         info.isRunning = getTopNonFinishingActivity() != null;
@@ -3298,7 +3303,7 @@
         if (mRootProcess != null) {
             pw.print(prefix); pw.print("mRootProcess="); pw.println(mRootProcess);
         }
-        pw.print(prefix); pw.print("taskId=" + mTaskId); pw.println(" stackId=" + getStackId());
+        pw.print(prefix); pw.print("taskId=" + mTaskId); pw.println(" stackId=" + getRootTaskId());
         pw.print(prefix + "hasBeenVisible=" + hasBeenVisible);
         pw.print(" mResizeMode=" + ActivityInfo.resizeModeToString(mResizeMode));
         pw.print(" mSupportsPictureInPicture=" + mSupportsPictureInPicture);
@@ -3315,7 +3320,7 @@
             sb.append(" U=");
             sb.append(mUserId);
             sb.append(" StackId=");
-            sb.append(getStackId());
+            sb.append(getRootTaskId());
             sb.append(" sz=");
             sb.append(getChildCount());
             sb.append('}');
@@ -3358,7 +3363,7 @@
         forAllActivities((r) -> {
             r.dumpDebug(proto, ACTIVITIES);
         });
-        proto.write(STACK_ID, getStackId());
+        proto.write(STACK_ID, getRootTaskId());
         if (mLastNonFullscreenBounds != null) {
             mLastNonFullscreenBounds.dumpDebug(proto, LAST_NON_FULLSCREEN_BOUNDS);
         }
diff --git a/services/core/java/com/android/server/wm/TaskChangeNotificationController.java b/services/core/java/com/android/server/wm/TaskChangeNotificationController.java
index 688fe12..2dde0ba 100644
--- a/services/core/java/com/android/server/wm/TaskChangeNotificationController.java
+++ b/services/core/java/com/android/server/wm/TaskChangeNotificationController.java
@@ -350,7 +350,7 @@
     void notifyActivityPinned(ActivityRecord r) {
         mHandler.removeMessages(NOTIFY_ACTIVITY_PINNED_LISTENERS_MSG);
         final Message msg = mHandler.obtainMessage(NOTIFY_ACTIVITY_PINNED_LISTENERS_MSG,
-                r.getTask().mTaskId, r.getStackId(), r.packageName);
+                r.getTask().mTaskId, r.getRootTaskId(), r.packageName);
         msg.sendingUid = r.mUserId;
         forAllLocalListeners(mNotifyActivityPinned, msg);
         msg.sendToTarget();
diff --git a/services/core/java/com/android/server/wm/WindowProcessController.java b/services/core/java/com/android/server/wm/WindowProcessController.java
index c755e60..812942e 100644
--- a/services/core/java/com/android/server/wm/WindowProcessController.java
+++ b/services/core/java/com/android/server/wm/WindowProcessController.java
@@ -618,15 +618,15 @@
 
         // Compare the z-order of ActivityStacks if both activities landed on same display.
         if (display == topDisplay
-                && mPreQTopResumedActivity.getActivityStack().compareTo(
-                        activity.getActivityStack()) <= 0) {
+                && mPreQTopResumedActivity.getRootTask().compareTo(
+                        activity.getRootTask()) <= 0) {
             canUpdate = true;
         }
 
         if (canUpdate) {
             // Make sure the previous top activity in the process no longer be resumed.
             if (mPreQTopResumedActivity != null && mPreQTopResumedActivity.isState(RESUMED)) {
-                final ActivityStack stack = mPreQTopResumedActivity.getActivityStack();
+                final ActivityStack stack = mPreQTopResumedActivity.getRootTask();
                 if (stack != null) {
                     stack.startPausingLocked(false /* userLeaving */, false /* uiSleeping */,
                             activity);
diff --git a/services/core/java/com/android/server/wm/WindowState.java b/services/core/java/com/android/server/wm/WindowState.java
index aef79a5..6173488 100644
--- a/services/core/java/com/android/server/wm/WindowState.java
+++ b/services/core/java/com/android/server/wm/WindowState.java
@@ -16,7 +16,7 @@
 
 package com.android.server.wm;
 
-import static android.app.ActivityTaskManager.INVALID_STACK_ID;
+import static android.app.ActivityTaskManager.INVALID_TASK_ID;
 import static android.app.AppOpsManager.MODE_ALLOWED;
 import static android.app.AppOpsManager.MODE_DEFAULT;
 import static android.app.AppOpsManager.OP_NONE;
@@ -1055,7 +1055,7 @@
                 }
             }
 
-            final ActivityStack stack = getStack();
+            final ActivityStack stack = getRootTask();
             if (inPinnedWindowingMode() && stack != null
                     && stack.lastAnimatingBoundsWasToFullscreen()) {
                 // PIP edge case: When going from pinned to fullscreen, we apply a
@@ -1469,12 +1469,10 @@
         return mActivityRecord != null ? mActivityRecord.getTask() : null;
     }
 
-    ActivityStack getStack() {
-        Task task = getTask();
+    ActivityStack getRootTask() {
+        final Task task = getTask();
         if (task != null) {
-            if (task.getStack() != null) {
-                return task.getStack();
-            }
+            return (ActivityStack) task.getRootTask();
         }
         // Some system windows (e.g. "Power off" dialog) don't have a task, but we would still
         // associate them with some stack to enable dimming.
@@ -2229,7 +2227,7 @@
             return false;
         }
 
-        final ActivityStack stack = getStack();
+        final ActivityStack stack = getRootTask();
         if (stack != null && stack.shouldIgnoreInput()) {
             // Ignore when the stack shouldn't receive input event.
             // (i.e. the minimized stack in split screen mode.)
@@ -2472,7 +2470,7 @@
             if (task != null) {
                 task.getDimBounds(mTmpRect);
             } else {
-                getStack().getDimBounds(mTmpRect);
+                getRootTask().getDimBounds(mTmpRect);
             }
         }
         if (inFreeformWindowingMode()) {
@@ -3563,12 +3561,12 @@
         return mTmpRect;
     }
 
-    private int getStackId() {
-        final ActivityStack stack = getStack();
+    private int getRootTaskId() {
+        final ActivityStack stack = getRootTask();
         if (stack == null) {
-            return INVALID_STACK_ID;
+            return INVALID_TASK_ID;
         }
-        return stack.mStackId;
+        return stack.mTaskId;
     }
 
     private void dispatchResized(Rect frame, Rect contentInsets,
@@ -3753,7 +3751,7 @@
         super.dumpDebug(proto, WINDOW_CONTAINER, logLevel);
         writeIdentifierToProto(proto, IDENTIFIER);
         proto.write(DISPLAY_ID, getDisplayId());
-        proto.write(STACK_ID, getStackId());
+        proto.write(STACK_ID, getRootTaskId());
         mAttrs.dumpDebug(proto, ATTRIBUTES);
         mGivenContentInsets.dumpDebug(proto, GIVEN_CONTENT_INSETS);
         mWindowFrames.dumpDebug(proto, WINDOW_FRAMES);
@@ -3795,10 +3793,9 @@
 
     @Override
     void dump(PrintWriter pw, String prefix, boolean dumpAll) {
-        final ActivityStack stack = getStack();
         pw.print(prefix + "mDisplayId=" + getDisplayId());
-        if (stack != null) {
-            pw.print(" stackId=" + stack.mStackId);
+        if (getRootTask() != null) {
+            pw.print(" rootTaskId=" + getRootTaskId());
         }
         pw.println(" mSession=" + mSession
                 + " mClient=" + mClient.asBinder());
@@ -5271,7 +5268,7 @@
             outPoint.offset(-parentBounds.left, -parentBounds.top);
         }
 
-        ActivityStack stack = getStack();
+        ActivityStack stack = getRootTask();
 
         // If we have stack outsets, that means the top-left
         // will be outset, and we need to inset ourselves
diff --git a/services/core/java/com/android/server/wm/WindowStateAnimator.java b/services/core/java/com/android/server/wm/WindowStateAnimator.java
index 6189fbd..069ee4f 100644
--- a/services/core/java/com/android/server/wm/WindowStateAnimator.java
+++ b/services/core/java/com/android/server/wm/WindowStateAnimator.java
@@ -1013,7 +1013,7 @@
                         mSurfaceController.deferTransactionUntil(mSurfaceController.mSurfaceControl,
                                 mWin.getFrameNumber());
                     } else {
-                        final ActivityStack stack = mWin.getStack();
+                        final ActivityStack stack = mWin.getRootTask();
                         mTmpPos.x = 0;
                         mTmpPos.y = 0;
                         if (stack != null) {
diff --git a/services/tests/wmtests/src/com/android/server/wm/ActivityRecordTests.java b/services/tests/wmtests/src/com/android/server/wm/ActivityRecordTests.java
index c60ca48..af3a3fe 100644
--- a/services/tests/wmtests/src/com/android/server/wm/ActivityRecordTests.java
+++ b/services/tests/wmtests/src/com/android/server/wm/ActivityRecordTests.java
@@ -136,7 +136,7 @@
     public void testStackCleanupOnTaskRemoval() {
         mStack.removeChild(mTask, null /*reason*/);
         // Stack should be gone on task removal.
-        assertNull(mService.mRootWindowContainer.getStack(mStack.mStackId));
+        assertNull(mService.mRootWindowContainer.getStack(mStack.mTaskId));
     }
 
     @Test
@@ -1136,7 +1136,7 @@
 
     @Test
     public void testRemoveFromHistory() {
-        final ActivityStack stack = mActivity.getActivityStack();
+        final ActivityStack stack = mActivity.getRootTask();
         final Task task = mActivity.getTask();
 
         mActivity.removeFromHistory("test");
diff --git a/services/tests/wmtests/src/com/android/server/wm/ActivityStarterTests.java b/services/tests/wmtests/src/com/android/server/wm/ActivityStarterTests.java
index 9e54f40..f702d5d 100644
--- a/services/tests/wmtests/src/com/android/server/wm/ActivityStarterTests.java
+++ b/services/tests/wmtests/src/com/android/server/wm/ActivityStarterTests.java
@@ -153,7 +153,7 @@
         assertThat((Object) task2.getStack()).isInstanceOf(ActivityStack.class);
         mStarter.updateBounds(task2, bounds);
 
-        verify(mService, times(1)).animateResizePinnedStack(eq(task2.getStack().mStackId),
+        verify(mService, times(1)).animateResizePinnedStack(eq(task2.getRootTaskId()),
                 eq(bounds), anyInt());
 
         // In the case of no animation, the stack and task bounds should be set immediately.
@@ -446,13 +446,13 @@
                 new ActivityBuilder(mService).setCreateTask(true).build();
         final ActivityRecord splitSecondReusableActivity =
                 new ActivityBuilder(mService).setCreateTask(true).build();
-        splitPrimaryFocusActivity.getActivityStack()
+        splitPrimaryFocusActivity.getRootTask()
                 .setWindowingMode(WINDOWING_MODE_SPLIT_SCREEN_PRIMARY);
-        splitSecondReusableActivity.getActivityStack()
+        splitSecondReusableActivity.getRootTask()
                 .setWindowingMode(WINDOWING_MODE_SPLIT_SCREEN_SECONDARY);
 
         // Set focus back to primary.
-        splitPrimaryFocusActivity.getActivityStack().moveToFront("testSplitScreenDeliverToTop");
+        splitPrimaryFocusActivity.getRootTask().moveToFront("testSplitScreenDeliverToTop");
 
         // Start activity and delivered new intent.
         starter.getIntent().setComponent(splitSecondReusableActivity.mActivityComponent);
@@ -477,18 +477,18 @@
                 new ActivityBuilder(mService).setCreateTask(true).build();
         final ActivityRecord splitPrimaryFocusActivity =
                 new ActivityBuilder(mService).setCreateTask(true).build();
-        splitPrimaryFocusActivity.getActivityStack()
+        splitPrimaryFocusActivity.getRootTask()
                 .setWindowingMode(WINDOWING_MODE_SPLIT_SCREEN_PRIMARY);
-        splitSecondReusableActivity.getActivityStack()
+        splitSecondReusableActivity.getRootTask()
                 .setWindowingMode(WINDOWING_MODE_SPLIT_SCREEN_SECONDARY);
-        splitSecondTopActivity.getActivityStack()
+        splitSecondTopActivity.getRootTask()
                 .setWindowingMode(WINDOWING_MODE_SPLIT_SCREEN_SECONDARY);
 
         // Make it on top of split-screen-secondary.
-        splitSecondTopActivity.getActivityStack().moveToFront("testSplitScreenTaskToFront");
+        splitSecondTopActivity.getRootTask().moveToFront("testSplitScreenTaskToFront");
 
         // Let primary stack has focus.
-        splitPrimaryFocusActivity.getActivityStack().moveToFront("testSplitScreenTaskToFront");
+        splitPrimaryFocusActivity.getRootTask().moveToFront("testSplitScreenTaskToFront");
 
         // Start activity and delivered new intent.
         starter.getIntent().setComponent(splitSecondReusableActivity.mActivityComponent);
@@ -732,13 +732,13 @@
         final ActivityRecord activity = new ActivityBuilder(mService)
                 .setCreateTask(true).build();
         new ActivityBuilder(mService)
-                .setStack(activity.getActivityStack())
+                .setStack(activity.getRootTask())
                 .setCreateTask(true).build();
 
         // Create a top finishing activity.
         final ActivityRecord finishingTopActivity = new ActivityBuilder(mService)
                 .setCreateTask(true).build();
-        finishingTopActivity.getActivityStack().moveToFront("finishingTopActivity");
+        finishingTopActivity.getRootTask().moveToFront("finishingTopActivity");
 
         assertEquals(finishingTopActivity, mRootWindowContainer.topRunningActivity());
         finishingTopActivity.finishing = true;
@@ -997,8 +997,8 @@
         assertThat(outActivity[0].inSplitScreenWindowingMode()).isFalse();
 
         // Move activity to split-screen-primary stack and make sure it has the focus.
-        top.getActivityStack().setWindowingMode(WINDOWING_MODE_SPLIT_SCREEN_PRIMARY);
-        top.getActivityStack().moveToFront("testWindowingModeOptionsLaunchAdjacent");
+        top.getRootTask().setWindowingMode(WINDOWING_MODE_SPLIT_SCREEN_PRIMARY);
+        top.getRootTask().moveToFront("testWindowingModeOptionsLaunchAdjacent");
 
         // Activity must landed on split-screen-secondary when launch adjacent.
         starter.setActivityOptions(options.toBundle())
diff --git a/services/tests/wmtests/src/com/android/server/wm/LaunchParamsControllerTests.java b/services/tests/wmtests/src/com/android/server/wm/LaunchParamsControllerTests.java
index cf1f0a8..8b3a6cf 100644
--- a/services/tests/wmtests/src/com/android/server/wm/LaunchParamsControllerTests.java
+++ b/services/tests/wmtests/src/com/android/server/wm/LaunchParamsControllerTests.java
@@ -284,7 +284,7 @@
 
         doNothing().when(mService).moveStackToDisplay(anyInt(), anyInt());
         mController.layoutTask(task, null /* windowLayout */);
-        verify(mService, times(1)).moveStackToDisplay(eq(task.getStackId()),
+        verify(mService, times(1)).moveStackToDisplay(eq(task.getRootTaskId()),
                 eq(params.mPreferredDisplayId));
     }
 
diff --git a/services/tests/wmtests/src/com/android/server/wm/RecentsAnimationTest.java b/services/tests/wmtests/src/com/android/server/wm/RecentsAnimationTest.java
index 7e42c62..ec8e402 100644
--- a/services/tests/wmtests/src/com/android/server/wm/RecentsAnimationTest.java
+++ b/services/tests/wmtests/src/com/android/server/wm/RecentsAnimationTest.java
@@ -97,7 +97,7 @@
                 .setStack(recentsStack)
                 .build();
         ActivityRecord topActivity = new ActivityBuilder(mService).setCreateTask(true).build();
-        topActivity.getActivityStack().moveToFront("testRecentsActivityVisiblility");
+        topActivity.getRootTask().moveToFront("testRecentsActivityVisiblility");
 
         doCallRealMethod().when(mRootWindowContainer).ensureActivitiesVisible(
                 any() /* starting */, anyInt() /* configChanges */,
@@ -187,7 +187,7 @@
         recentActivity.app = null;
 
         // Start an activity on top.
-        new ActivityBuilder(mService).setCreateTask(true).build().getActivityStack().moveToFront(
+        new ActivityBuilder(mService).setCreateTask(true).build().getRootTask().moveToFront(
                 "testRestartRecentsActivity");
 
         doCallRealMethod().when(mRootWindowContainer).ensureActivitiesVisible(
@@ -229,7 +229,7 @@
         new ActivityBuilder(mService)
                 .setCreateTask(true)
                 .build()
-                .getActivityStack()
+                .getRootTask()
                 .moveToFront("Activity start");
 
         // Start the recents animation.
diff --git a/services/tests/wmtests/src/com/android/server/wm/RootActivityContainerTests.java b/services/tests/wmtests/src/com/android/server/wm/RootActivityContainerTests.java
index ea8d082..07dfc50 100644
--- a/services/tests/wmtests/src/com/android/server/wm/RootActivityContainerTests.java
+++ b/services/tests/wmtests/src/com/android/server/wm/RootActivityContainerTests.java
@@ -638,7 +638,7 @@
         final ActivityRecord resolverActivity = mRootWindowContainer.topRunningActivity();
 
         assertEquals(info, resolverActivity.info);
-        assertEquals(ACTIVITY_TYPE_STANDARD, resolverActivity.getActivityStack().getActivityType());
+        assertEquals(ACTIVITY_TYPE_STANDARD, resolverActivity.getRootTask().getActivityType());
     }
 
     /**
diff --git a/services/tests/wmtests/src/com/android/server/wm/TaskPositionerTests.java b/services/tests/wmtests/src/com/android/server/wm/TaskPositionerTests.java
index 8970571..7aaf3fb 100644
--- a/services/tests/wmtests/src/com/android/server/wm/TaskPositionerTests.java
+++ b/services/tests/wmtests/src/com/android/server/wm/TaskPositionerTests.java
@@ -85,7 +85,7 @@
 
         mPositioner.register(mDisplayContent, win);
 
-        win.getStack().setWindowingMode(WINDOWING_MODE_FREEFORM);
+        win.getRootTask().setWindowingMode(WINDOWING_MODE_FREEFORM);
     }
 
     @After
diff --git a/services/tests/wmtests/src/com/android/server/wm/WindowStateTests.java b/services/tests/wmtests/src/com/android/server/wm/WindowStateTests.java
index 6d0b54f..5cf9c44 100644
--- a/services/tests/wmtests/src/com/android/server/wm/WindowStateTests.java
+++ b/services/tests/wmtests/src/com/android/server/wm/WindowStateTests.java
@@ -278,7 +278,7 @@
         spyOn(stack);
         doReturn(true).when(controller).isMinimizedDock();
         doReturn(true).when(controller).isHomeStackResizable();
-        doReturn(stack).when(appWindow).getStack();
+        doReturn(stack).when(appWindow).getRootTask();
 
         // Make sure canBeImeTarget is false due to shouldIgnoreInput is true;
         assertFalse(appWindow.canBeImeTarget());