When single-line and double-line borders meet (e.g., a Level 1 panel inside a Level 3 dialog), use mixed junction characters:

GlyphUnicodeDescription
U+255FDouble vertical, single right
U+2562Double vertical, single left
U+2564Single vertical, double horizontal (top)
U+2567Single vertical, double horizontal (bottom)
U+256BDouble vertical, single horizontal cross
U+256ASingle vertical, double horizontal cross
U+2553Double vertical, single horizontal (down-right)
U+2556Double vertical, single horizontal (down-left)
U+2559Double vertical, single horizontal (up-right)
U+255CDouble vertical, single horizontal (up-left)