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(0, 0, 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(0, 0, element.Width, element.Height);
                    popPatientInfos.PlacementRectangle = rtWnd;
                    break;
                }
            }
            while (parent != null);
        }