https://learn.microsoft.com/ja-jp/dotnet/api/system.windows.controls.contextmenu.placementtarget?view=netframework-4.8.1&viewFallbackFrom=net-5.0
ContextMenu.PlacementTarget Property (System.Windows.Controls) | Microsoft Learn
Gets or sets the UIElement relative to which the ContextMenu is positioned when it opens.
property systemcontextmenuwindowscontrolsmicrosoft