20 20 78 78 78 ... Looks ASCII to me, Firefox on Windows. Could be OS.

" xxx"? That's the same in ASCII and UTF-8.

OP is asking what are the line-drawing characters encoded as e.g: "┌" and "┐".

Since the charset returned by the app is UTF-8, these will be interpreted and encoded as UTF-8 and not whatever "ASCII - Extended" means.

Firefox on Linux: I just copied and pasted into emacs and did a M-x describe-char and got 0xE29480 which is definitely not ASCII: https://www.compart.com/en/unicode/U+2500

Also confirmed with hexl-mode (hex editor in emacs)