I love how compact this code is, and how it relies on nothing other than css!
Some kind of visible "affordance-cue" seems needed, to invite the hovering. Very slightly tweaked (without any delay), I like:
```
<style>
.pop div {display:none; ; background-color:#ffd;}
.pop:hover {background-color:#ffa; border-right: 6px solid #ffa;}
.pop:hover div {display:block;}
</style>
<$list filter="[tag[Articles]]">
<div class="pop" style="border-top: 6px solid #ffd; border-left: 6px solid #ffd;">
{{!!title}}
<div style="border-left:2em solid #ffa; padding-left:1em; max-height:100px;overflow:auto;">
<$transclude mode="block">
</div></div>
</$list>
```