For external links, I managed to achieve this with a CSS snippet. It’s not perfect (especially for long URIs or link text) but for me it’s working nicely:
a.external-link {
position: relative;
}
a.external-link:before {
position: absolute;
padding: 0px 8px 0px 8px;
color: var(--text-normal);
background-color: var(--background-primary-alt);
border-radius: 3px;
font-size: 80%;
display: none;
z-index: 1000;
top: 1.8em;
content: attr(href)
}
a.external-link:hover:before {
display: inline;
}