We have helped ourselves by adding the following CSS:
@media print {
body[ng-app='SOGo.SchedulerUI'] div.md-dialog-container {
display: flex !important;
position: fixed;
align-items: baseline;
}
body[ng-app='SOGo.SchedulerUI'] div.md-dialog-container md-dialog-actions,
body[ng-app='SOGo.SchedulerUI'].md-dialog-is-showing main section div div.view-detail,
body[ng-app='SOGo.SchedulerUI'].md-dialog-is-showing main section div div.view-list md-content md-tabs {
display: none !important;
}
}

Now you can print a single appointment when the appropriate window is open.