You have the :hover pseudo on your CSS. While this exists, the behaviour persists.
You can add a .small-window class to your <html> tag and control it with that.
https://jsfiddle.net/9a1n8xtw/4/
$(window).on('resize', function() {
    isSmallWindow = $(this).width() < 768;
    if(isSmallWindow) {
        $('html').addClass('small-window');
    }else {
        $('html').removeClass('small-window');
    }
});
And the css
html:not(.small-window) .headline:hover + .content {
    display: block;
}