On the classic forge trove select, you can pick Topic and UI selections at any level, not just leaf nodes. Allura requires you to select a leaf node. For example you can't pick "Topic :: Internet :: WWW/HTTP :: Browsers" you must pick "Topic :: Internet :: WWW/HTTP :: Browsers :: Plugins and add-ons"
It appears that on classic forge, this is only true for certain categories. License and audience, for example, require you to pick a leaf node.
I'd be fine with changing the UI to make this happen. Personally I think it's easier to browse through a single drop-down like classic does.
forge:tv/3345
First run:
Then browse to the trove categories page and make sure adding and deleting still works, and that the Topic and UI lists now contain non-leaf troves that can be selected and added.
Looks good, tested fine, merged to dev.