C# WPF RibbonApplicationMenu无法设置下拉高度

在WPF应用程序中使用Microsoft Ribbon控件并使用应用程序菜单，如下所示：

<r:Ribbon.ApplicationMenu>
    <r:RibbonApplicationMenu
        <r:RibbonApplicationSplitMenuItem Header="Item 1">
            <r:RibbonApplicationSplitMenuItem.Items>
                <r:RibbonApplicationMenuItem Header="Sub Item 1" />
                <r:RibbonApplicationMenuItem Header="Sub Item 2" />
                <r:RibbonApplicationMenuItem Header="Sub Item 3" />
                <r:RibbonApplicationMenuItem Header="Sub Item 4" />
                <r:RibbonApplicationMenuItem Header="Sub Item 5" />
                <r:RibbonApplicationMenuItem Header="Sub Item 6" />
                <r:RibbonApplicationMenuItem Header="Sub Item 7" />
                <r:RibbonApplicationMenuItem Header="Sub Item 8" />
                <r:RibbonApplicationMenuItem Header="Sub Item 9" />
            </r:RibbonApplicationSplitMenuItem.Items>
        </r:RibbonApplicationSplitMenuItem>
        <r:RibbonApplicationSplitMenuItem Header="Item 2" />
    </r:RibbonApplicationMenu>
</r:Ribbon.ApplicationMenu>