Comment 2 for bug 1414315

If you have a lot of categories, I suggest you not spend time scrolling
through the list looking for individual ones, instead use the find at
the top of the tag browser, that auto-expands the parent category when
it finds a match.

Auto-expanding everything with a large library will simply make the widget
unmangeable, with too many entries to scroll through.

That said, if you really, really want the ability to expand a particular
node and all its descendants, I can add it to the right click menu. If
so, feel free to reply.

 status wontfix