IIUC fixed widths is more than just a UI problem. Lots of internal code is assuming that. But maybe it works well-enough that it isn't a real issue.
I remember trying a terminal that didn't have fixed char widths and it mostly worked. The only real issue was line wrapping (as vim and the terminal would both wrap based on how long they considered the line to be). So maybe it would be possible! I would love to see solid support for variable-width fonts.
I know variable-width fonts are controversial, but I actually think they work well for coding but haven't had the chance to use them much due to Vim limitations. But especially for Markdown and similar mostly-text content they seem like a clear win.
