div.content {
    height: 0pt;
    visibility:hidden;
    opacity:0;
    overflow:hidden;
    transition: visibility 0s, opacity 0.5s linear;
}

div.content.visible,
div.content.mention {
    height:100%;
    visibility:visible;
    opacity:1;
    overflow:auto;
}
h1.visible_content::before {
    content: "▼ ";
}
h1::before {
    content: "► ";
}
h1.title.mention::before {
    content:"";
}