Make Panel scroll the correct div (#12736)

Redefined util method to take the scrollable instead of its parent to make
it useful also for cases where the first child is not scrollable

Change-Id: I301d260d26373b38fdee525f169347bb7b4c499e

client/src/com/vaadin/client/Util.java

@@ -525,12 +525,11 @@ public class Util {
      * See: bugs #11994 and #12736.
      * 
      * @param contentNode
-     *            an element that contains a scrollable element as its first
-     *            child
+     *            an element that is scrollable
      * 
      * @since 7.1.8
      */
-    public static void removeUnneededScrollbars(final Element contentNode) {
+    public static void removeUnneededScrollbars(final Element scrollable) {
         if (BrowserInfo.get().isWebkit()) {
 
             /*
@@ -544,9 +543,6 @@ public class Util {
 
                 @Override
                 public void execute() {
-                    final com.google.gwt.dom.client.Element scrollable = contentNode
-                            .getFirstChildElement();
-
                     // Adjusting the width or height may change the scroll
                     // position, so store the current position
                     int horizontalScrollPosition = scrollable.getScrollLeft();

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

@@ -343,7 +343,8 @@ public class VWindow extends VOverlay implements ShortcutActionHandlerOwner,
             super.setVisible(visible);
         }
         if (visible && BrowserInfo.get().isWebkit()) {
-            Util.removeUnneededScrollbars(contents);
+            Util.removeUnneededScrollbars((Element) contents
+                    .getFirstChildElement());
             updateContentsSize();
             positionOrSizeUpdated();
         }