I found an intertesting DCS behavior:
1. I have the DCS screen shows on the external display (DCS resolution is the exact the externall display's resolution, no flipMode = false in autoexec.cfg)
2. the edge of the DCS app widget shows on the my primary screen (I just noticed it), and shut DCS down.
3. after apply "flipMode = false" in autoexec.cfg, launch DCS again with applied "Alt+Enter"
The edge of widget is gone, and I am wondering that the now DCS app is running "full-screen".
Thanks.