Fix incorrect position of column selector in MPR (#11953)

Cherry pick from
https://github.com/vaadin/framework/pull/11946

client/src/main/java/com/vaadin/client/ui/VContextMenu.java

 import com.google.gwt.core.client.Scheduler;
 import com.google.gwt.dom.client.Element;
 import com.google.gwt.dom.client.NodeList;
+import com.google.gwt.dom.client.Style;
 import com.google.gwt.dom.client.TableRowElement;
 import com.google.gwt.dom.client.TableSectionElement;
 import com.google.gwt.event.dom.client.BlurEvent;
                 setHeight(Window.getClientHeight() + "px");
             }
             setPopupPosition(menuLeft, menuTop);
+            getElement().getStyle().setPosition(Style.Position.FIXED);
 
             /*
              * Move keyboard focus to menu, deferring the focus setting so the

compatibility-client/src/main/java/com/vaadin/v7/client/ui/VScrollTable.java

         public void onBrowserEvent(Event event) {
             if (enabled) {
                 if (event.getEventTarget().cast() == columnSelector) {
-                    final int left = DOM.getAbsoluteLeft(columnSelector);
-                    final int top = DOM.getAbsoluteTop(columnSelector)
+                    WidgetUtil.TextRectangle clientRect = WidgetUtil
+                            .getBoundingClientRect(columnSelector);
+                    final int left = (int) clientRect.getLeft();
+                    final int top = (int) (clientRect.getTop()
                             + DOM.getElementPropertyInt(columnSelector,
-                                    "offsetHeight");
+                                    "offsetHeight"));
                     client.getContextMenu().showAt(this, left, top);
                 }
             }