This seems to be an unpopular opinion, but proportional fonts are great.
One question people sometimes ask is, "How can you code in a proportional font? How do you line things up in columns?"
The answer is you don't. For example, the Black code formatter for Python completely eschews column alignment of this sort:
foo.bar(one,
two,
three)
(Imagine that the words were long enough that the whole thing wouldn't fit on one line.)
Instead, Black formats it like this:
foo.bar(
one,
two,
three
)
No column alignment, only indentation. Code formatted like this is equally readable in a proportional or monospaced font. You don't need monospaced fonts!
I've read and written all my code in proportional fonts for close to 20 years. My current one is Trebuchet++, a variant of Trebuchet MS that I customized in FontForge.
Trebuchet MS is pretty good on its own. For example, all of the "confusable" glyphs are easy to distinguish. And it renders very nicely on a high-DPI display.
Its tilde is pretty bad, though. It's tiny and almost looks like a hyphen. So that was the first thing I fixed, putting in a better tilde.
Then I experimented with spacing for the _ and . glyphs. I added negative margin on both sides of the _, while keeping its width. And I added positive spacing on both sides of the . glyph. Consider code like this:
snake_case_name.another_snaky_name()
These spacing changes have the affect of pulling the individual words in each name closer together, while increasing the separation between the two names.
It is subtle, but to my eye it helps readability.
Next up will be to add some positive margin inside the three pairs of brackety things: (), [], {}. I have always found that it helps readability to add a space inside parens and such, but most contemporary code formatters prohibit this. With a proportional font I can tweak the font to put some visual spacing there.