WPF popup被截断的原因和修改方法

原因：wpf里 popup不能超过屏幕75%的面积，不知道为什么要这么设置？

修改方法：

 private void SetPopupScreen()
         {
             Rect rtWnd = new Rect(, , gridMainArea.ActualWidth, gridMainArea.ActualHeight);
             DependencyObject parent = popPatientInfos.Child;
             do
             {
                 parent = VisualTreeHelper.GetParent(parent);
                 if (parent != null && parent.ToString() == "System.Windows.Controls.Primitives.PopupRoot")
                 {
                     var element = parent as FrameworkElement;
                     element.Width = gridMainArea.ActualWidth;//根据需要修改宽高
                     element.Height = gridMainArea.ActualHeight;
                     rtWnd = new Rect(, , element.Width, element.Height);
                     popPatientInfos.PlacementRectangle = rtWnd;
                     break;
                 }
             }
             while (parent != null);
         }