Also hide shim iframe of VOverlay on setVisible(false) (#12731)

Change-Id: I48258a24bb14bbe79c1e11c9bda4a18b120b2a42

client/src/com/vaadin/client/ui/VOverlay.java

@@ -229,6 +229,10 @@ public class VOverlay extends PopupPanel implements CloseHandler<PopupPanel> {
         return shadow != null;
     }
 
+    protected boolean isShimElementEnabled() {
+        return shimElement != null;
+    }
+
     private void removeShimElement() {
         if (shimElement != null) {
             shimElement.removeFromParent();
@@ -382,6 +386,10 @@ public class VOverlay extends PopupPanel implements CloseHandler<PopupPanel> {
             shadow.getStyle().setProperty("visibility",
                     visible ? "visible" : "hidden");
         }
+        if (isShimElementEnabled()) {
+            shimElement.getStyle().setProperty("visibility",
+                    visible ? "visible" : "hidden");
+        }
     }
 
     @Override