Issue with diagonal box-drawing characters

[Logo121]

Currently the diagonal box-drawing characters ╱╲╳ has a slight vertical (only) overshoot like │.
On the other hand, the "middle diagonal" box-drawing characters like 🮦🮧 do not overshoot, making the two (sets of) diagonals actually have slightly different slope.

Since there are characters in #2189 like

that are combinations of these 2 types of diagonals, their geometry probably need to be unified some way.

(also 🮦🮧 etc are clipped beyond their bounding boxes while ╳ etc aren't, which is probably another discrepancy to be addressed)