Improve transition to PiP mode.

Use setAutoEnterEnable to true for smooth transition to
Picture-in-Picture when in gestural navigation mode.

Closes #12878

app/src/main/java/org/thoughtcrime/securesms/WebRtcCallActivity.java

@@ -125,6 +125,7 @@ public class WebRtcCallActivity extends BaseActivity implements SafetyNumberChan
   private WindowLayoutInfoConsumer         windowLayoutInfoConsumer;
   private WindowInfoTrackerCallbackAdapter windowInfoTrackerCallbackAdapter;
   private ThrottledDebouncer               requestNewSizesThrottle;
+  private PictureInPictureParams.Builder   pipBuilderParams;
 
   private Disposable ephemeralStateDisposable = Disposable.empty();
 
@@ -156,6 +157,7 @@ public void onCreate(Bundle savedInstanceState) {
 
     initializeResources();
     initializeViewModel(isLandscapeEnabled);
+    initializePictureInPictureParams();
 
     processIntent(getIntent());
 
@@ -275,14 +277,16 @@ public void onBackPressed() {
   }
 
   private boolean enterPipModeIfPossible() {
-    if (viewModel.canEnterPipMode() && isSystemPipEnabledAndAvailable()) {
-      PictureInPictureParams params = new PictureInPictureParams.Builder()
-          .setAspectRatio(new Rational(9, 16))
-          .build();
-      enterPictureInPictureMode(params);
-      CallParticipantsListDialog.dismiss(getSupportFragmentManager());
+    if (isSystemPipEnabledAndAvailable()) {
+      if (viewModel.canEnterPipMode()) {
+        enterPictureInPictureMode(pipBuilderParams.build());
+        CallParticipantsListDialog.dismiss(getSupportFragmentManager());
 
-      return true;
+        return true;
+      }
+      if (Build.VERSION.SDK_INT >= 31) {
+        pipBuilderParams.setAutoEnterEnabled(false);
+      }
     }
     return false;
   }
@@ -362,6 +366,19 @@ private void initializeViewModel(boolean isLandscapeEnabled) {
     });
   }
 
+  private void initializePictureInPictureParams() {
+    if (isSystemPipEnabledAndAvailable()) {
+      pipBuilderParams = new PictureInPictureParams.Builder();
+      pipBuilderParams.setAspectRatio(new Rational(9, 16));
+      if (Build.VERSION.SDK_INT >= 31) {
+        pipBuilderParams.setAutoEnterEnabled(true);
+      }
+      if (Build.VERSION.SDK_INT >= 26) {
+        setPictureInPictureParams(pipBuilderParams.build());
+      }
+    }
+  }
+
   private void handleViewModelEvent(@NonNull WebRtcCallViewModel.Event event) {
     if (event instanceof WebRtcCallViewModel.Event.StartCall) {
       startCall(((WebRtcCallViewModel.Event.StartCall) event).isVideoCall());