confirmation on form close

Referencing https://datatables.net/forums/discussion/32883, I tried this implementation. I see (at least on Chrome) that if I use ESC, the popup is displayed twice, but Cancel does indeed bring user back to form to continue editing. If I close the form using the X in the upper right, however, the form closes whether I click OK or Cancel.

Is there something I'm doing wrong? I can set up test area for you again if needed.

In case there is some conflict specific to my processing, my full event handler setup is

// this must be done after datatables() is called in datatables.js
var openVals;
function afterdatatables(){
    editor.on( 'uploadXhrSuccess', function ( e, fieldName, json ) {
        console.log ('elev = ' + json.elev + ' distance = ' + json.distance);
        editor.field('elev').set(json.elev);
        editor.field('distance').set(json.distance);
        editor.field('active').set(json.active);
        editor.field('location').set(json.location);
    } );

    editor.on('initCreate', function() {
        editor.set('active', 1)
        editor.field('active').hide()
    });

    editor.on('initEdit', function() {
        var fileid = editor.get('fileid');
        editor.set('turns', 'Loading...')
        editor.field('active').show()
        $.ajax({
            // rr_turns_url_prefix comes from runningroute-*-config.js
            url: rr_turns_url_prefix + '/admin/' + fileid + '/turns',
            success : function(data) {
                editor.set('turns', data.turns)
            },
            error : function(jqXHR, textStatus, errorThrown) {
                editor.set('turns', 'ERROR: could not retrieve turn data\n'
                            + '   ' + errorThrown)
            },
        });

    });

    // confirm before closing window for unsaved changes
    // from https://datatables.net/forums/discussion/32883
    editor
        .on( 'open', function () {
            // Store the values of the fields on open
            openVals = JSON.stringify( editor.get() );
        } )
       .on( 'preClose', function ( e ) {
            // On close, check if the values have changed and ask for closing confirmation if they have
            if ( openVals !== JSON.stringify( editor.get() ) ) {
                return confirm( 'You have unsaved changes. Are you sure you want to exit?' );
            }
        } )
       .on( 'preBlur', function ( e ) {
            // On close, check if the values have changed and ask for closing confirmation if they have
            if ( openVals !== JSON.stringify( editor.get() ) ) {
                return confirm( 'You have unsaved changes. Are you sure you want to exit?' );
            }
    } );
};