MenuItem with SubmenuHeaderTemplate - Popup invalid positioning

Describe the bug

The placement for the sub-items in the ContextMenu works incorrectly if the popup is displayed above the parent control. This happens because of the DropShadowEffect that is added to the child item popup and requires some offsets and margins to be aligned. This works fine in cases when the popup is placed below the parent element.

To Reproduce

I've added the following ContextMenu to the Gallery app NotifyIcon:

   <tray:NotifyIcon
        Grid.Row="0"
        FocusOnLeftClick="True"
        Icon="pack://application:,,,/Assets/wpfui.png"
        MenuOnRightClick="True"
        TooltipText="WPF UI Gallery">
        <tray:NotifyIcon.Menu>
            <ContextMenu>
                <MenuItem Header="Item 1">
                    <MenuItem Header="Item 1.1"/>
                    <MenuItem Header="Item 1.2"/>
                </MenuItem>
                <MenuItem Header="Item 2">
                    <MenuItem Header="Item 2.1"/>
                    <MenuItem Header="Item 2.2"/>
                </MenuItem>
                <Separator/>
                <MenuItem Header="Item 3">
                </MenuItem>
            </ContextMenu>
        </tray:NotifyIcon.Menu>
    </tray:NotifyIcon>

Expected behavior

The popup should be placed right next to the MenuItem. We can achieve that by removing the DropShadowEffect and setting the VerticalOffset and Margin to 0

Screenshots

OS version

Win 11 24H2

.NET version

8.0

WPF-UI NuGet version

latest main branch

Additional context

No response

Start with the Gallery app NotifyIcon ContextMenu reproduction and inspect the MenuItem submenu popup styling, especially its DropShadowEffect, VerticalOffset, and Margin. Compare positioning when the popup appears above and below the parent MenuItem. Done means the submenu popup sits directly beside its parent in both placements without the reported offset.