Function to 'pop up' a new frame;

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make-frame returns the frame that was just made. Capture that frame in a let form and pass it onto toggle-frame-fullscreen. (let ((frame (make-frame))) ... ;; do thingy (toggle-frame-fullscreen frame))

You might also be interested in the built-in support emacs has for using new frames more often and new windows (inside an existing frame — what some people would call “use new graphical windows instead of dividing windows up into sub-windows”) more often. The easy way into this is to look at the documentation for pop-up-frames inside emacs itself. Hope this helps!