hi,
a bit late but who knows 😉
i am no jquery master but this work for me, if it’s ok, it would be great to have it merged (hey devs), btw it’s working great on wp 4.7.4

// When the document is ready...
jQuery( document ).ready( function( $ ) {

    // ... display the Full Screen search when:
    // 1. The user focuses on a search field, or
    // 2. The user clicks the Search button
    $( 'form[role=search] input, form[role=search] button' ).on( 'focus, click', function( event ) {
        // Prevent the default action
        event.preventDefault();

        // Display the Full Screen Search
        $( '#full-screen-search' ).addClass( 'open' );

        // Focus on the Full Screen Search Input Field
        $( '#full-screen-search input' ).focus();
    } );

    // Hide the Full Screen search when the user clicks the close button
    $( '#full-screen-search button.close' ).on( 'click', function( event ) {
        // Prevent the default event
        event.preventDefault();

        // Hide the Full Screen Search
        $( '#full-screen-search' ).removeClass( 'open' );
    } );

	//ADD THIS CODE FOR ESC KEY
	$(document).keypress(function(e) {if (e.keyCode==27){ $( '#full-screen-search' ).removeClass( 'open' );}});

} );