Box Drawings Double Up and Left

It closes the bottom-right of a double-line box. Put it at the end of the bottom double line, directly under the right-hand double line.

It comes from 1980s text-mode computing. The IBM PC's built-in character set, code page 437, included single and double line pieces so programs could draw windows, menus and tables on screens that could only show text. Unicode later took these pieces over, which is why the same corner still turns up in terminal programs and DOS-style art.

How do I keep a text box aligned?

Build the whole box from the double-line set: ═ for horizontal runs, ║ for sides, and ╔ ╗ ╚ ╝ for corners. Mixing in single-line pieces leaves visible joins that don't meet.

Use a monospaced font, where every character has the same width, or a code block. In a proportional font, a missing glyph or a terminal with different character widths, the lines drift apart even when the characters are correct.