[ui] fix fade-out in loading decorator with smooth scrolling enabled

In order to have smooth scrolling, loading layer should not be on top of viewport, but rather added on demand.
But since loading layer has fade-out, it can not be removed immediately when setLoadingLayerVisible is called.
It should be removed only at the end of fade out.
Before that removal, it should paint with zero alpha, in order not to leave artifacts after itself.

platform/platform-api/src/com/intellij/openapi/ui/LoadingDecorator.java

@@ -66,7 +66,8 @@ public class LoadingDecorator {
 
       @Override
       protected void paintCycleEnd() {
-        setLoadingLayerVisible(false);
+        myLoadingLayer.setAlpha(0); // paint with zero alpha before hiding completely
+        hideLoadingLayer();
         myLoadingLayer.setAlpha(-1);
       }
     };
@@ -81,16 +82,16 @@ public class LoadingDecorator {
     Disposer.register(parent, myLoadingLayer.myProgress);
   }
 
-  private void setLoadingLayerVisible(boolean visible) {
+  private void hideLoadingLayer() {
     if (SystemProperties.isTrueSmoothScrollingEnabled()) {
-      setLoadingLayerPresent(visible);
+      myPane.remove(myLoadingLayer);
     }
-    myLoadingLayer.setVisible(visible);
+    myLoadingLayer.setVisible(false);
   }
 
   public void setLoadingLayerVisible(boolean visible, boolean takeSnapshot) {
-    if (SystemProperties.isTrueSmoothScrollingEnabled()) {
-      setLoadingLayerPresent(visible);
+    if (SystemProperties.isTrueSmoothScrollingEnabled() && visible) {
+      setLoadingLayerPresent();
     }
     myLoadingLayer.setVisible(visible, takeSnapshot);
   }
@@ -102,13 +103,9 @@ public class LoadingDecorator {
 
      Blit-acceleration copies as much of the rendered area as possible and then repaints only newly exposed region.
      This helps to improve scrolling performance and to reduce CPU usage (especially if drawing is compute-intensive). */
-  private void setLoadingLayerPresent(boolean present) {
-    if (present) {
-      if (myPane.getComponentCount() < 2) {
-        myPane.add(myLoadingLayer, JLayeredPane.DRAG_LAYER, 1);
-      }
-    } else {
-      myPane.remove(myLoadingLayer);
+  private void setLoadingLayerPresent() {
+    if (myPane.getComponentCount() < 2) {
+      myPane.add(myLoadingLayer, JLayeredPane.DRAG_LAYER, 1);
     }
   }