Improve onFirstContentfulPaintnotification

app/src/common/shared/org/mozilla/vrbrowser/browser/engine/Session.java

@@ -466,6 +466,7 @@ private void closeSession(@NonNull SessionState aState) {
         }
         aState.mSession.close();
         aState.setActive(false);
+        mFirstContentfulPaint = false;
     }
 
     public void captureBitmap() {
@@ -1053,6 +1054,14 @@ public void onFirstComposite(@NonNull GeckoSession aSession) {
             for (GeckoSession.ContentDelegate listener : mContentListeners) {
                 listener.onFirstComposite(aSession);
             }
+            if (mFirstContentfulPaint) {
+                // onFirstContentfulPaint is only called once after a session is opened.
+                // Notify onFirstContentfulPaint after a session is reattached before
+                // being closed ((e.g. tab selected)
+                for (GeckoSession.ContentDelegate listener : mContentListeners) {
+                    listener.onFirstContentfulPaint(aSession);
+                }
+            }
         }
     }