I used two different borders, one for the "Default" decor and another one for "Titlebarless" decor. If I switch off the titlebar the "Titlebarless" decor comes but if something update the window title (here it's my zsh which update my terminal) then it switches back to the "Default" decor.

You can try the theme here: http://box-look.org/content/show.php?content=104039

Try something which can update your window's title.