It looks fine in chat or in a `field[]` or `textlist[]`, but in hypertext there's quite noticeable gap between any corner and the vertical pipe its connected to
Yes, this is a limitation due to the vertical spacing between font lines in the engine. I've ran into the same issue for my insidethebox server, which uses this exact sort of ascii-art bordering for all of its dialog windows.
You can opt to instead draw actual lines, through, and overlay them.