bug#52067: Acknowledgement (29.0.50; string-glyph-split halts on certain emoji strings)

[Paul Maragakis via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

The logic in string-glyph-split expects the first two elements in the result
from find-composition-internal to give the start and end of a multibyte grapheme
and return nil when there is a regular character at position POS.  However, this 
isn't always the case.

Let's call x the argument POS in find-composition-internal, 
and "interval" the first two elements of the return value.

The following example works as expected, i.e. x of 0, or 1 returns the interval (0 2), 
and x of 2, or 3 returns (2 4).

(null
 (pp
  (mapcar '(lambda (x) (list x (find-composition-internal x nil "✈️✈️" nil))) '(0 1 2 3 4))))
((0
  (0 2
     [[#<font-object "-*-Apple Color Emoji-medium-normal-normal-*-19-*-*-*-m-0-iso10646-1"> 9992 65039]
      296
      [0 1 9992 233 23 0 23 18 4 nil]]))
 (1
  (0 2
     [[#<font-object "-*-Apple Color Emoji-medium-normal-normal-*-19-*-*-*-m-0-iso10646-1"> 9992 65039]
      296
      [0 1 9992 233 23 0 23 18 4 nil]]))
 (2
  (2 4
     [[#<font-object "-*-Apple Color Emoji-medium-normal-normal-*-19-*-*-*-m-0-iso10646-1"> 9992 65039]
      296
      [0 1 9992 233 23 0 23 18 4 nil]]))
 (3
  (2 4
     [[#<font-object "-*-Apple Color Emoji-medium-normal-normal-*-19-*-*-*-m-0-iso10646-1"> 9992 65039]
      296
      [0 1 9992 233 23 0 23 18 4 nil]]))
 (4 nil))
nil


In the following case, however, x of 2 returns interval (0 2).

(null
 (pp
  (mapcar '(lambda (x) (list x (find-composition-internal x nil "✈️🌍" nil))) '(0 1 2 3))))
((0
  (0 2
     [[#<font-object "-*-Apple Color Emoji-medium-normal-normal-*-19-*-*-*-m-0-iso10646-1"> 9992 65039]
      296
      [0 1 9992 233 23 0 23 18 4 nil]]))
 (1
  (0 2
     [[#<font-object "-*-Apple Color Emoji-medium-normal-normal-*-19-*-*-*-m-0-iso10646-1"> 9992 65039]
      296
      [0 1 9992 233 23 0 23 18 4 nil]]))
 (2
  (0 2
     [[#<font-object "-*-Apple Color Emoji-medium-normal-normal-*-19-*-*-*-m-0-iso10646-1"> 9992 65039]
      296
      [0 1 9992 233 23 0 23 18 4 nil]]))
 (3 nil))
nil


Interestingly, in the following case, an x of 0, 1, 2, or 3 all return (0 2).

(null
 (pp
  (mapcar '(lambda (x) (list x (find-composition-internal x nil "✈️🌍🌍" nil))) '(0 1 2 3 4))))
((0
  (0 2
     [[#<font-object "-*-Apple Color Emoji-medium-normal-normal-*-19-*-*-*-m-0-iso10646-1"> 9992 65039]
      296
      [0 1 9992 233 23 0 23 18 4 nil]]))
 (1
  (0 2
     [[#<font-object "-*-Apple Color Emoji-medium-normal-normal-*-19-*-*-*-m-0-iso10646-1"> 9992 65039]
      296
      [0 1 9992 233 23 0 23 18 4 nil]]))
 (2
  (0 2
     [[#<font-object "-*-Apple Color Emoji-medium-normal-normal-*-19-*-*-*-m-0-iso10646-1"> 9992 65039]
      296
      [0 1 9992 233 23 0 23 18 4 nil]]))
 (3
  (0 2
     [[#<font-object "-*-Apple Color Emoji-medium-normal-normal-*-19-*-*-*-m-0-iso10646-1"> 9992 65039]
      296
      [0 1 9992 233 23 0 23 18 4 nil]]))
 (4 nil))
nil


And in the following case a POS of 3 returns (3 5)

(null
 (pp
  (mapcar '(lambda (x) (list x (find-composition-internal x nil "✈️🌍✈️" nil))) '(0 1 2 3 4 5))))
((0
  (0 2
     [[#<font-object "-*-Apple Color Emoji-medium-normal-normal-*-19-*-*-*-m-0-iso10646-1"> 9992 65039]
      296
      [0 1 9992 233 23 0 23 18 4 nil]]))
 (1
  (0 2
     [[#<font-object "-*-Apple Color Emoji-medium-normal-normal-*-19-*-*-*-m-0-iso10646-1"> 9992 65039]
      296
      [0 1 9992 233 23 0 23 18 4 nil]]))
 (2
  (0 2
     [[#<font-object "-*-Apple Color Emoji-medium-normal-normal-*-19-*-*-*-m-0-iso10646-1"> 9992 65039]
      296
      [0 1 9992 233 23 0 23 18 4 nil]]))
 (3
  (3 5
     [[#<font-object "-*-Apple Color Emoji-medium-normal-normal-*-19-*-*-*-m-0-iso10646-1"> 9992 65039]
      296
      [0 1 9992 233 23 0 23 18 4 nil]]))
 (4
  (3 5
     [[#<font-object "-*-Apple Color Emoji-medium-normal-normal-*-19-*-*-*-m-0-iso10646-1"> 9992 65039]
      296
      [0 1 9992 233 23 0 23 18 4 nil]]))
 (5 nil))
nil


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