Bug 1449567 - Don't reattach compositor for the same compositor object; r=snorp

If we're trying to detach and reattach the same compositor object for
whatever reason, we should skip it so we don't inadvertently end up not
attaching the object at all.

Differential Revision: https://phabricator.services.mozilla.com/D5608

--HG--
extra : moz-landing-system : lando

mobile/android/geckoview/src/main/java/org/mozilla/gecko/gfx/LayerSession.java

@@ -76,7 +76,7 @@ public class LayerSession {
         @WrapForJNI(calledFrom = "ui", dispatchTo = "gecko")
         @Override protected native void disposeNative();
 
-        @WrapForJNI(calledFrom = "any", dispatchTo = "gecko")
+        @WrapForJNI(calledFrom = "ui", dispatchTo = "gecko")
         public native void attachNPZC(PanZoomController npzc);
 
         @WrapForJNI(calledFrom = "ui", dispatchTo = "gecko")

widget/android/nsWindow.cpp

@@ -1239,13 +1239,16 @@ nsWindow::GeckoViewSupport::Transfer(const GeckoSession::Window::LocalRef& inst,
         window.mNPZCSupport.Detach();
     }
 
-    if (window.mLayerViewSupport) {
+    if (window.mLayerViewSupport &&
+            window.mLayerViewSupport->GetJavaCompositor() != aCompositor) {
         window.mLayerViewSupport.Detach();
     }
 
     auto compositor = LayerSession::Compositor::LocalRef(
             inst.Env(), LayerSession::Compositor::Ref::From(aCompositor));
-    window.mLayerViewSupport.Attach(compositor, &window, compositor);
+    if (!window.mLayerViewSupport) {
+        window.mLayerViewSupport.Attach(compositor, &window, compositor);
+    }
 
     MOZ_ASSERT(window.mAndroidView);
     window.mAndroidView->mEventDispatcher->Attach(