This is not as bad an idea as it would seem, but it's never going to happen with Unicode. It could reasonably happen with a combination of NerdFonts / private area shenanigans + cell rendering on the terminal side of this to handle it.

It solves the z-index problem neatly for an individual cell as you just render in order (first underneath, second over the top). But it doesn't solve the order for adjacent cells - borders that lie between cells. At least not easily. Not an insurmountable problem.

Some perspective if you're unaware. Fonts have the unicode line drawing characters, but fonts don't align super well and so a good terminal will draw those characters with line drawing algorithms instead of as a font. I've got a WIP PR that I need to land in VSCode's that pulls in xtermjs and ghostty's line drawing stuff so that it fixes up all the existing gaps on that as an example of this problem.