その問題はまだ表立って議論されていない。
The issue hasn't been discussed publicly yet.
内密に means 'secretly/confidentially', the direct opposite of 表立って, which describes doing something openly.
表面 is a plain noun meaning 'surface/outward appearance', while 表立って is an adverbial expression describing the manner of an action being public.
Standard kanji spelling; commonly used in news and formal contexts.
Variant spelling with だ in kana; less common but still encountered.
Derived from 表 (おもて, 'front, public') + 立つ (たつ, 'to stand') in its て-form, literally 'standing in the open'. The exact historical development is uncertain, but the expression has been used since at least the Edo period.