If 50% of the work when creating a app is spent on the display manager your probably doing something wrong or linux is a way wrost of a shape then i thought.
My understanding is the display manager is useless all you require from the display manager is the window position, drag events, resizes, mouse/keyboard input and maybe display resolution correct??
the display should not even try to mess with whats inside your app (should not draw anything in your app).
I will read that article you sent more detailed when i have a bit more "study time" but all these things can and should be done by the toolkit quiet easly.
a stupid example for creating a window:
function create_window ();
Funtion call_toolkitapi (name)
print ("sorry this display manager is not supported")
This is how i understand how it works (kinda)...
i will study it and maybe even try can create an app for linux just to have some facts.