The editor/word processor built-in to the Psion s3c and later pdas in the early nineties supported proportional font editing of code with indents preserved by treating tabs not as a series of spaces, but as physical locations like a word processor does.

Most programming language layout patterns only use tabs to indent up to the first character of text.

I use Slickedit with a proportional font and only in the rarest of cases does it cause a problem even without the physical tabbing the Psion had. Using a proportional font makes code so much more compact and easier to read.