What I didn't say in my original comment is that I don't actually do this for exactly the reason you describe, and that I think spaces-only is the most robust s

What I didn't say in my original comment is that I don't actually do this for exactly the reason you describe, and that I think spaces-only is the most robust solution. That said, all you would need is editors to enforce "any line with spaces before a non-whitespace character must have exactly the same number of tabs as the previous line." Which would not be all that difficult, but currently (afaik) does not exist.