메인 채팅과 스레드 사이의 창 구분선 - 조정할 수 없음

That’s incorrect, this commit didn’t do this. It’s been introduced in UX: update minwidth to chat sidepanel + better title word-break fn (#… · discourse/discourse@cdf8eb8 · GitHub

Anyways I already reverted it few days ago: FIX: lower thread panel min-width (#30158) · discourse/discourse@b0be89c · GitHub