TSF/IME question. (programming issue)

Question

I am currently developing a ThirdParty Hangul IME using Microsoft text service framework (aka TSF).

I have one problem, which I am not able to solve.

I have 2 questions.

1. TSF related question. ITfLangBarItemButton::InitMenu(ITfMenu *pMenu) is not called at all. (Of course, TF_LANGBARITEMINFO.dwStyle contains TF_LBI_STYLE_BTN_MENU). I wonder if this is intentional behavior.
2. the attached picture shows the popup menu that appears when you right-click on the LanguageBar Button of the Windows default Hangul IME. (I'm guessing this Popup menu is the WinUI3 menuFlyout) Which process is responsible for displaying this menu on the screen? I would like to implement a similar popup menu in my IME. I am assuming that this is done in a windowless background process that displays the menuFlyout on right-click on LanugageBar button. However, I have realized that it is impossible to display the WinUI3 menuFlyout in a windowless background process. Which programming model should I apply?

PS.
I can't find the appropriate tag, so if this is the wrongime_menu.png, please forward it to the appropriate person.