x-display-backing-store

x-display-backing-store is a built-in function in `C source code'.

(x-display-backing-store &optional TERMINAL)

Return an indication of whether X display TERMINAL does backing store.
The value may be `always', `when-mapped', or `not-useful'.
The optional argument TERMINAL specifies which display to ask about.
TERMINAL should be a terminal object, a frame or a display name (a string).
If omitted or nil, that stands for the selected frame's display.