diff --git a/services/core/java/com/android/server/wm/WindowState.java b/services/core/java/com/android/server/wm/WindowState.java
index af8c4c8e93701..1ba036e646950 100644
--- a/services/core/java/com/android/server/wm/WindowState.java
+++ b/services/core/java/com/android/server/wm/WindowState.java
@@ -5485,17 +5485,19 @@ class WindowState extends WindowContainer<WindowState> implements WindowManagerP
     }
 
     private void updateScaleIfNeeded() {
-        if (mIsChildWindow) {
-            // Child window follows parent's scale.
-            return;
-        }
         if (!isVisibleRequested() && !(mIsWallpaper && mToken.isVisible())) {
             // Skip if it is requested to be invisible, but if it is wallpaper, it may be in
             // transition that still needs to update the scale for zoom effect.
             return;
         }
-        float newHScale = mHScale * mGlobalScale * mWallpaperScale;
-        float newVScale = mVScale * mGlobalScale * mWallpaperScale;
+        float globalScale = mGlobalScale;
+        final WindowState parent = getParentWindow();
+        if (parent != null) {
+            // Undo parent's scale because the child surface has inherited scale from parent.
+            globalScale *= parent.mInvGlobalScale;
+        }
+        final float newHScale = mHScale * globalScale * mWallpaperScale;
+        final float newVScale = mVScale * globalScale * mWallpaperScale;
         if (mLastHScale != newHScale || mLastVScale != newVScale) {
             getSyncTransaction().setMatrix(mSurfaceControl, newHScale, 0, 0, newVScale);
             mLastHScale = newHScale;
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 446ec8b2680bc..334cfd5fbb8fd 100644
--- a/services/tests/wmtests/src/com/android/server/wm/WindowStateTests.java
+++ b/services/tests/wmtests/src/com/android/server/wm/WindowStateTests.java
@@ -664,12 +664,11 @@ public class WindowStateTests extends WindowTestsBase {
         assertEquals(expectedChildPos, childPos);
 
         // Surface should apply the scale.
+        final SurfaceControl.Transaction t = w.getPendingTransaction();
         w.prepareSurfaces();
-        verify(w.getPendingTransaction()).setMatrix(w.getSurfaceControl(),
-                overrideScale, 0, 0, overrideScale);
+        verify(t).setMatrix(w.mSurfaceControl, overrideScale, 0, 0, overrideScale);
         // Child surface inherits parent's scale, so it doesn't need to scale.
-        verify(child.getPendingTransaction(), never()).setMatrix(any(), anyInt(), anyInt(),
-                anyInt(), anyInt());
+        verify(t, never()).setMatrix(any(), anyInt(), anyInt(), anyInt(), anyInt());
 
         // According to "dp * density / 160 = px", density is scaled and the size in dp is the same.
         final CompatibilityInfo compatInfo = cmp.compatibilityInfoForPackageLocked(
@@ -686,6 +685,13 @@ public class WindowStateTests extends WindowTestsBase {
         final Rect unscaledClientBounds = new Rect(clientConfig.windowConfiguration.getBounds());
         unscaledClientBounds.scale(overrideScale);
         assertEquals(w.getWindowConfiguration().getBounds(), unscaledClientBounds);
+
+        // Child window without scale (e.g. different app) should apply inverse scale of parent.
+        doReturn(1f).when(cmp).getCompatScale(anyString(), anyInt());
+        final WindowState child2 = createWindow(w, TYPE_APPLICATION_SUB_PANEL, "child2");
+        clearInvocations(t);
+        child2.prepareSurfaces();
+        verify(t).setMatrix(child2.mSurfaceControl, w.mInvGlobalScale, 0, 0, w.mInvGlobalScale);
     }
 
     @UseTestDisplay(addWindows = {W_ABOVE_ACTIVITY, W_NOTIFICATION_SHADE})