diff --git a/core/java/android/app/WindowConfiguration.java b/core/java/android/app/WindowConfiguration.java
index 4ae1670e90412..c4af4edb467b3 100644
--- a/core/java/android/app/WindowConfiguration.java
+++ b/core/java/android/app/WindowConfiguration.java
@@ -796,6 +796,9 @@ public class WindowConfiguration implements Parcelable, Comparable<WindowConfigu
     /**
      * Returns {@code true} if the windowingMode represents a window in multi-window mode.
      * I.e. sharing the screen with another activity.
+     *
+     * TODO(b/171672645): This API could be misleading - in 'undefined' mode it's determined by the
+     * parent's mode
      * @hide
      */
     public static boolean inMultiWindowMode(int windowingMode) {
diff --git a/services/core/java/com/android/server/wm/ActivityTaskManagerService.java b/services/core/java/com/android/server/wm/ActivityTaskManagerService.java
index 48b0b7d279178..153204528b8d0 100644
--- a/services/core/java/com/android/server/wm/ActivityTaskManagerService.java
+++ b/services/core/java/com/android/server/wm/ActivityTaskManagerService.java
@@ -39,6 +39,7 @@ import static android.app.WindowConfiguration.WINDOWING_MODE_SPLIT_SCREEN_PRIMAR
 import static android.app.WindowConfiguration.WINDOWING_MODE_UNDEFINED;
 import static android.content.Intent.FLAG_ACTIVITY_EXCLUDE_FROM_RECENTS;
 import static android.content.Intent.FLAG_ACTIVITY_NEW_TASK;
+import static android.content.pm.ActivityInfo.RESIZE_MODE_UNRESIZEABLE;
 import static android.content.pm.ApplicationInfo.FLAG_FACTORY_TEST;
 import static android.content.pm.ConfigurationInfo.GL_ES_VERSION_UNDEFINED;
 import static android.content.pm.PackageManager.FEATURE_ACTIVITIES_ON_SECONDARY_DISPLAYS;
@@ -1294,6 +1295,7 @@ public class ActivityTaskManagerService extends IActivityTaskManager.Stub {
         a.screenOrientation = ActivityInfo.SCREEN_ORIENTATION_UNSPECIFIED;
         a.colorMode = ActivityInfo.COLOR_MODE_DEFAULT;
         a.flags |= ActivityInfo.FLAG_EXCLUDE_FROM_RECENTS;
+        a.resizeMode = RESIZE_MODE_UNRESIZEABLE;
 
         final ActivityOptions options = ActivityOptions.makeBasic();
         options.setLaunchActivityType(ACTIVITY_TYPE_DREAM);
diff --git a/services/core/java/com/android/server/wm/DisplayContent.java b/services/core/java/com/android/server/wm/DisplayContent.java
index 4a30ca96abb6f..8c5f52bc3b9df 100644
--- a/services/core/java/com/android/server/wm/DisplayContent.java
+++ b/services/core/java/com/android/server/wm/DisplayContent.java
@@ -4997,6 +4997,13 @@ class DisplayContent extends RootDisplayArea implements WindowManagerPolicy.Disp
                 || windowingMode == WINDOWING_MODE_MULTI_WINDOW);
     }
 
+    static boolean canReuseExistingTask(int windowingMode, int activityType) {
+        // Existing Tasks can be reused if a new stack will be created anyway, or for the Dream -
+        // because there can only ever be one DreamActivity.
+        return alwaysCreateStack(windowingMode, activityType)
+                || activityType == ACTIVITY_TYPE_DREAM;
+    }
+
     @Nullable
     Task getFocusedStack() {
         return getItemFromTaskDisplayAreas(TaskDisplayArea::getFocusedStack);
diff --git a/services/core/java/com/android/server/wm/Task.java b/services/core/java/com/android/server/wm/Task.java
index ecee46e895d0e..f64047d0e8a3b 100644
--- a/services/core/java/com/android/server/wm/Task.java
+++ b/services/core/java/com/android/server/wm/Task.java
@@ -2813,16 +2813,25 @@ class Task extends WindowContainer<WindowContainer> {
 
         int windowingMode =
                 getResolvedOverrideConfiguration().windowConfiguration.getWindowingMode();
+        final int parentWindowingMode = newParentConfig.windowConfiguration.getWindowingMode();
 
         // Resolve override windowing mode to fullscreen for home task (even on freeform
         // display), or split-screen if in split-screen mode.
         if (getActivityType() == ACTIVITY_TYPE_HOME && windowingMode == WINDOWING_MODE_UNDEFINED) {
-            final int parentWindowingMode = newParentConfig.windowConfiguration.getWindowingMode();
             windowingMode = WindowConfiguration.isSplitScreenWindowingMode(parentWindowingMode)
                     ? parentWindowingMode : WINDOWING_MODE_FULLSCREEN;
             getResolvedOverrideConfiguration().windowConfiguration.setWindowingMode(windowingMode);
         }
 
+        // Do not allow non-resizable non-pinned tasks to be in a multi-window mode - they should
+        // use their parent's windowing mode, or fullscreen.
+        if (!isResizeable() && windowingMode != WINDOWING_MODE_PINNED
+                && WindowConfiguration.inMultiWindowMode(windowingMode)) {
+            windowingMode = WindowConfiguration.inMultiWindowMode(parentWindowingMode)
+                    ? WINDOWING_MODE_FULLSCREEN : parentWindowingMode;
+            getResolvedOverrideConfiguration().windowConfiguration.setWindowingMode(windowingMode);
+        }
+
         if (isLeafTask()) {
             resolveLeafOnlyOverrideConfigs(newParentConfig, mTmpBounds /* previousBounds */);
         }
@@ -3312,7 +3321,9 @@ class Task extends WindowContainer<WindowContainer> {
     }
 
     boolean isResizeable(boolean checkSupportsPip) {
-        return (mAtmService.mForceResizableActivities || ActivityInfo.isResizeableMode(mResizeMode)
+        final boolean forceResizable = mAtmService.mForceResizableActivities
+                && getActivityType() == ACTIVITY_TYPE_STANDARD;
+        return (forceResizable || ActivityInfo.isResizeableMode(mResizeMode)
                 || (checkSupportsPip && mSupportsPictureInPicture));
     }
 
@@ -7174,7 +7185,7 @@ class Task extends WindowContainer<WindowContainer> {
             ActivityRecord source, ActivityOptions options) {
 
         Task task;
-        if (DisplayContent.alwaysCreateStack(getWindowingMode(), getActivityType())) {
+        if (DisplayContent.canReuseExistingTask(getWindowingMode(), getActivityType())) {
             // This stack will only contain one task, so just return itself since all stacks ara now
             // tasks and all tasks are now stacks.
             task = reuseAsLeafTask(voiceSession, voiceInteractor, intent, info, activity);