Yes, I was thinking to this when I've read the OP. I have source code I wrote in the 90's that was displaying text boxes with the border on the actual border of

Yes, I was thinking to this when I've read the OP. I have source code I wrote in the 90's that was displaying text boxes with the border on the actual border of the characters, without any color bleeding. But I was just using the normal mode (256 characters, just redefining some in the upper 128 block). On VGA, by default only characters in a specific range can be used for horizontal line drawing, as these are the ones that get the 9th bit extended... I've actually reused this code a few months ago for a prank (UEFI network booted Linux containing DosBox running fullscreen in the framebuffer, to display a fake OS installer).