x-display-save-under

x-display-save-under is a built-in function in `C source code'.

(x-display-save-under &optional TERMINAL)

Return t if the X display TERMINAL supports the save-under feature.
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.