elgavon This is a bit clumsy, because modal-fullscreen only applies for text-based files that open in the code editor. By design, it does not logically apply for any other non-text file types. Besides, when in modal-fullscreen, you can't navigate between modal items. Therefore, even if this was to be an option, it would be specific for text/code files, and it would effectively block navigation between modal items when enabled.
Instead of editing the Javascript, if you want to hardcode text/code files to display in modal-fullscreen, you could add some custom CSS _files/css/custom.css
.
.modal-popup-code {
--width: auto;
--justify: stretch;
--modal-pos-button: none;
--align: stretch;
--border-radius: 0;
--margin: 0px;
--modal-nav-display: none;
--fullscreen-collapse: block;
--fullscreen-expand: none;
}