Handle only bg or fg set when resolving cursor hl

[mmrwoods]

The previous code to resolve the cursor highlight didn't allow for only
the background or foreground to be set, i.e. only one of guifg & guibg.

This was done intentionally to keep the code simple, on the assumption
that only setting either background or foreground was so rare that it
didn't warrant the additional complexity, but it seems that assumption
was misguided, while it is unusual, it's not rare enough to ignore.

See #84 for background

Thanks @casr for reporting this and researching in some detail :-)

[mmrwoods]

@casr Could you test these changes when you get a chance? I think/hope they fix the issue.

[casr]

Thank you! I'll have a play.

On first glance it seems more complex than I thought it would turn out but I might need to catch up to where your thinking is at! I'll try and set up some tests across term, cterm, gui and termguicolors with different combinations of fg/bg. Hopefully that'll be useful to test against.

It might take me a couple of days to get to the time together though if you don't mind a short wait.

[mmrwoods]

The complexity seems to be unavoidable as fg/gb/foreground/background are not valid values for ctermfg and ctermbg, and in terminal vim we need to allow for users switching between termguicolors and notermguicolors (I do this myself at times) which means all of guifg, guibg, ctermfg, and ctermbg must be set. It would be lovely to just set the gui fg and bg and set the cterm cursor to reverse, but that mucks up the termguicolors cursor.

[mmrwoods]

The simplest option here would be just to always use the fallback (term=reverse cterm=reverse) in non-gui vim, which doesn't seem at all that unreasonable, but is a change to the existing behaviour.

[mmrwoods]

btw, re fg/bg/foregound/background not being valid ctermfg and ctermbg values, this is not always the case, :hi foo ctermfg=bg ctermbg=fg actually works fine if Vim knows the normal foreground and background colors, but that requires the Normal hlgroup to be set, which is not always the case.

Having said that, this can be identified by checking for specific error numbers (see :h ctermfg), so maybe that is an option to simplify.

[casr]

This works from my testing 🎉

---

I did notice one quirk - separate from this - where if you load a vim session without choosing any colorscheme you get an error when launching something like :FuzzyFiles. I think it's because none of the default named colours exist until after a colorscheme is chosen so the devicons highlight groups cannot be assigned. It's a really rare edge case though so probably not worth worrying about!

[mmrwoods]

Thanks for testing, and noting that bug re devicon colors, I can reproduce locally using a minimal vimrc and vim -u min.vim -S Session.vim. Will investigate, and open an issue to track if not easy to fix.

[mmrwoods]

bug re devicon colors fixed, see e5ebb44