bug#27403: 26.0.50; Indentation misalignment with Unicode code points >65535

[Eli Zaretskii <eliz@gnu.org>]

> From: Adam Niederer <adam.niederer@gmail.com>
> Date: Sat, 17 Jun 2017 02:28:41 -0400
> 
> Hello, I believe I've found an indentation issue. To reproduce, start
> emacs, create a buffer in js-mode, paste in this code, and press C-x h
> TAB to indent the buffer:
> 
> let x = /* 👍 */ { foo: 0
>                    bar: 0 }
> 
> let x = /* ☺ */ { foo: 0
>                   bar: 0 }
> 
> Both 25.2 and 26.0.50 add one extra space before "bar" in the first
> first snippet with U+1F44D THUMBS UP SIGN in the comment, whereas the
> second snippet with U+263A WHITE SMILING FACE properly aligns "bar" with
> "foo".

That's because U+1F44D is a double-width character:

  (char-width ?👍) => 2

while U+263A is not double-width.

So as long as indentation works in columns and not in pixels, this is
a "feature".

> This appears to happen whenever the character in the comment needs a
> surrogate pair.

I don't believe surrogates have anything to do with this, since Emacs
works with Unicode codepoints, not their UTF-16 encodings.

> Interestingly, pressing TAB with one's point on the second line of each
> snippet to dedent the line yields a correct result for both symbols:
> 
> "👍", {"a": 2,
>     "b": 3}
> 
> "☺", {"a":2,
>     "b":3}

Which is probably a subtle bug: this should behave like the first
snippet.

Thanks.