This still isn't quite right...I found code like this will cause issues (1) and (2) to occur:

render() {
  return (
    <Item thing={<OtherItem field={something} />} />;
  );
}

Also, are the colors supposed to be the same as an XML String, or the same as Javascript code? I would expect it to render like Javascript, but that's not the case before or after this patch (before the patch it was rendering like an attribute, and after it's rendering like a string).

Right—this change is broken with any nontrivial amount of nesting of blocks and brace-enclosed strings. I'm not familiar with neovim or its paren-matching algorithms, so I can't speak to (2).

(3) is a legitimate issue with this approach, I believe (though I'm not fully parsing your sentence describing it). I'm not sure what the right approach is to fix it, however. If you come up with something, feel free to update this PR or submit a new one.

Regarding the comment on the colors (3), I just noticed I had written an incomplete sentence, so I edited that. Also, I figure some pictures might help illustrate what I'm trying to say.

Before the patch, an xml string ("string") is one color. For the javascript attributes, { is one color (the same color as a string), and jsVariable} is another color (the same color as the attribute name itself - in this case, data-javascript-variable):

After the patch, the full {jsVariable} portion is colored the same as "string". This seems more correct than before the patch:

I feel like the best result however would be for { and } to be the same color as "string", and the inner portion (jsVariable) to be javascript colored (offwhite in this colorscheme). Here's an edited screenshot of what that would look like:

Or another option would be for {jsVariable} to be colored as if it were plain javascript (offwhite in this colorscheme), like in this mockup:

FWIW I'm using gruvbox as a colorscheme. I tried a few other themes to see if I could get something a little more clear than two colors of green, but it looks like quite a few render strings and attributes the same color.