Fix Package Manager loading spinner

Editor/Core/UIElements/LoadingSpinner.cs

@@ -25,12 +25,22 @@ public LoadingSpinner()
             innerElement = new VisualElement();
             innerElement.AddToClassList("loading-spinner");
             innerElement.AddToClassList("hidden");
+#if UNITY_2021_2_OR_NEWER
+            // In Unity 2021.2 and later, transform origin of elements changed from
+            // top-left to center by default.
+            // https://discussions.unity.com/t/introducing-transform-styles/861646
+            // To maintain the same behavior as before, we set the transform origin to top-left.
+            style.transformOrigin = new TransformOrigin(Length.Percent(0), Length.Percent(0), 0);
+#endif
+            SetLoadingSpinnerBackground();
+
             Add(innerElement);
         }
 
         private void UpdateProgress()
         {
             transform.rotation = Quaternion.Euler(0, 0, m_Rotation);
+
             m_Rotation += 3;
             if (m_Rotation > 360)
                 m_Rotation -= 360;
@@ -59,5 +69,18 @@ public void Stop()
             started = false;
             innerElement.AddToClassList("hidden");
         }
+
+        private void SetLoadingSpinnerBackground()
+        {
+            // Unity 2019.1+ includes this as a loaded resource, but in earlier versions
+            // we need to load it from the Unity Package Manager package's resources.
+#if UNITY_2019_1_OR_NEWER
+            innerElement.style.backgroundImage = new StyleBackground(
+                (Texture2D)EditorGUIUtility.Load("icons/packagemanager/dark/loading.png")
+            );
+#else
+            innerElement.style.backgroundImage = AssetDatabase.LoadAssetAtPath<Texture2D>("Packages/com.unity.package-manager-ui/Editor/Resources/Images/Dark/loading.png");
+#endif
+        }
     }
 }

UXML/PackageManager/PackageSource.uss

@@ -10,7 +10,6 @@ LoadingSpinner{
 
 }
 .loading-spinner {
-    background-image: resource("icons/packagemanager/dark/loading.png");
 	top: -8px;
 	left: -8px;
 	width: 16px;