Nice!
Very nice and clean.
Just this week I started to use details as sections for articles. I use simple transclusion of the details tiddler in the article
tiddler, and with a new button I create the details tiddler. Mine works when exporting statics, something the DetailsWidget can't do, since widgets don't export into static html, and I need to publish articles as static html.
Here is my new button text (which also makes the summary use the tiddler title):
<$button class="tc-btn-invisible"><$action-sendmessage $message="tm-new-tiddler" text="<details><summary><b><$view field='title'/></b></summary><span>
your text here
</span></details>" />{{$:/core/images/new-button}}</$button>
Obviously, your solution is more apt for most use cases. I am definitelty going to experiment with it, and add it to the toolmap. Mine is specifically for transcluding sections of a longer article, but as details elements. Enjoy.