Does editor font have to be monospace?

The answer is no. And as usual I learned this after 20 years of programming. Although in the first years most editors were text based so the only font type was monospace font. I've changed my editor font to Franklin Gothic Demi Cond and just look at it:

It's much better than monospace font. Monospace may be required in some languages but in C++ it doesn't matter, because tab spacing has no real meaning. Besides as you can see tabs wont disappear.

I think (and many others also do) monospace fonts are less readable than regular (proportional) fonts. Selecting more readable font is important, because it also increases the readablity of the source code.