Calculate the correct surface layers for finishTransaction

We normally prevent assignLayers from touching surface z-order
during transition animations. However, this causes a problem
when multiple transitions happen in quick succession (eg. during
CTS tests) because final surface z-order may not be updated.

To fix this, allow assignLayers to set layers into the
finishTransaction. This is applied by shell and is expected
to represent the "z-order at the end of animation", so we
obviously should allow it to get the fully calculated layers.

Bug: 237545923
Test: atest PinnedStackTests
Change-Id: Ifb8c339fcd9fce7d3457dae806b6d3b186ea5d5c

services/core/java/com/android/server/wm/Transition.java

@@ -533,9 +533,14 @@ class Transition extends Binder implements BLASTSyncEngine.TransactionReadyListe
         }
         // Need to update layers on involved displays since they were all paused while
         // the animation played. This puts the layers back into the correct order.
-        for (int i = displays.size() - 1; i >= 0; --i) {
-            if (displays.valueAt(i) == null) continue;
-            displays.valueAt(i).assignChildLayers(t);
+        mController.mBuildingFinishLayers = true;
+        try {
+            for (int i = displays.size() - 1; i >= 0; --i) {
+                if (displays.valueAt(i) == null) continue;
+                displays.valueAt(i).assignChildLayers(t);
+            }
+        } finally {
+            mController.mBuildingFinishLayers = false;
         }
         if (rootLeash.isValid()) {
             t.reparent(rootLeash, null);

services/core/java/com/android/server/wm/TransitionController.java

@@ -100,6 +100,14 @@ class TransitionController {
     // TODO(b/188595497): remove when not needed.
     final StatusBarManagerInternal mStatusBar;
 
+    /**
+     * `true` when building surface layer order for the finish transaction. We want to prevent
+     * wm from touching z-order of surfaces during transitions, but we still need to be able to
+     * calculate the layers for the finishTransaction. So, when assigning layers into the finish
+     * transaction, set this to true so that the {@link canAssignLayers} will allow it.
+     */
+    boolean mBuildingFinishLayers = false;
+
     TransitionController(ActivityTaskManagerService atm,
             TaskSnapshotController taskSnapshotController,
             TransitionTracer transitionTracer) {
@@ -309,6 +317,15 @@ class TransitionController {
         return false;
     }
 
+    /**
+     * Whether WM can assign layers to window surfaces at this time. This is usually false while
+     * playing, but can be "opened-up" for certain transition operations like calculating layers
+     * for finishTransaction.
+     */
+    boolean canAssignLayers() {
+        return mBuildingFinishLayers || !isPlaying();
+    }
+
     @WindowConfiguration.WindowingMode
     int getWindowingModeAtStart(@NonNull WindowContainer wc) {
         if (mCollectingTransition == null) return wc.getWindowingMode();

services/core/java/com/android/server/wm/WindowContainer.java

@@ -2473,7 +2473,7 @@ class WindowContainer<E extends WindowContainer> extends ConfigurationContainer<
     void assignLayer(Transaction t, int layer) {
         // Don't assign layers while a transition animation is playing
         // TODO(b/173528115): establish robust best-practices around z-order fighting.
-        if (mTransitionController.isPlaying()) return;
+        if (!mTransitionController.canAssignLayers()) return;
         final boolean changed = layer != mLastLayer || mLastRelativeToLayer != null;
         if (mSurfaceControl != null && changed) {
             setLayer(t, layer);