Add code to yourextra.lesstemplate:
Code:
/* Hide user extra info */
.message-userExtras {
opacity: 0;
max-height: 0px;
overflow: hidden;
transition: all 0.5s ease-in-out;
}
.message-cell.message-cell--user:hover .message-userExtras {
opacity: 1;
max-height: 300px;
}
/**********/