間違い、cssだな。アドオンで手軽にできる時代はとうの昔に終わった

/* サイドバー自動開閉 */
#sidebar-box {
position: relative !important;
overflow-x: hidden !important;
margin-right: -15px !important;
left: 15px !important;
min-width: 15px !important;
max-width: 15px !important;
box-shadow: 0 0 2px 0 rgba(0,0,0,.35);
}
#sidebar-box:hover {
margin-right: -200px !important;
left: 200px !important;
min-width: 200px !important;
max-width: 200px !important;
box-shadow: 0 0 5px 0 rgba(0,0,0,.35);
}
#sidebar {
opacity: 0 !important;
}
#sidebar:hover {
opacity: 1 !important;
}
#sidebar-header {
display: none !important;
}
#sidebar-splitter {
}