https://learn.microsoft.com/en-us/dotnet/api/system.windows.controls.menuitem.submenuopened?view=netframework-4.8.1&viewFallbackFrom=windowsdesktop-10.0
MenuItem.SubmenuOpened Event (System.Windows.Controls) | Microsoft Learn
Occurs when the state of the IsSubmenuOpen property changes to true.
event systemmenuitemwindowscontrolsmicrosoft