Actually, Unicode "Symbols for Legacy Computing", U+1FB7C – U+1FB7F ('LEFT AND LOWER ONE EIGHTH BLOCK', 'LEFT AND UPPER ONE EIGHTH BLOCK', 'RIGHT AND UPPER ONE EIGHTH BLOCK', 'RIGHT AND LOWER ONE EIGHTH BLOCK'). But support may vary, as this is a relatively recent addition (based on a proposal from 2019 [1]).
That said, there is more to this approach, as it actually solves two problems at once: corners and the surrounding white space required for boxes with a background fill. And there are no compatibility requirements (as compared to using the new Unicode range.)
Actually, Unicode "Symbols for Legacy Computing", U+1FB7C – U+1FB7F ('LEFT AND LOWER ONE EIGHTH BLOCK', 'LEFT AND UPPER ONE EIGHTH BLOCK', 'RIGHT AND UPPER ONE EIGHTH BLOCK', 'RIGHT AND LOWER ONE EIGHTH BLOCK'). But support may vary, as this is a relatively recent addition (based on a proposal from 2019 [1]).
That said, there is more to this approach, as it actually solves two problems at once: corners and the surrounding white space required for boxes with a background fill. And there are no compatibility requirements (as compared to using the new Unicode range.)
[1] http://www.unicode.org/L2/L2019/19025-terminals-prop.pdf