Well, making it only appear when the mouse pointer is over the invisible
box might be tricky (and not very user-friendly).
However, you could make it disappear when the mouse pointer is not
hovering over the respective *tiddler* - try adding the following code
to your StyleSheet (shadow) tiddler:
.tiddler .tagged {
visibility: hidden;
}
.selected .tagged {
visibility: visible;
}
(You could also try "display: none/block;" instead of "visibility:
hidden/visible;" - this usually doesn't make much of a difference though.)
HTH.
-- F.