implementing tab-to-spacification in a way that requires tabs to work "as tabs" (i.e. you get your alignment along 4-space lines) with a mono-spaced font.

I suppose some terminal rendering (where you're taking in chunks and you might want to be a bit clever about jumping to spaces to figure out where breaks should go).

There is a notion of UTF codepoints that you can index into (see how Python does string indexing), though I generally think that people who whine a lot about string behavior for UTF tend to just not be reaching for the right tools.