bug#36006: electric-pair-mode fails to balance in certain cases

[Gregory Heytings]

I don't know if the following information is useful, but on the "<div>" 
example given by the OP, the difference between Emacs 26 and 27 is that, 
in `electric-pair--balance-info', in the expression

(eq (char-after pos) (electric-pair--with-uncached-syntax (table) 
(matching-paren (char-before (scan-sexps (point) 1)))))

the second argument fails with "Unbalanced parentheses" in Emacs 27, which 
means that the whole expression fails, whereas in Emacs 26 both arguments 
evaluate to 60 and the expression returns t.

Because of that `electric-pair--balance-info' returns ((nil . 60) nil . 
60) in Emacs 27, instead of ((t . 60) t) in Emacs 26.

Because of that `electric-pair-skip-if-helps-balance' and 
`electric-pair-default-skip-self' return nil in Emacs 27, instead of t in 
Emacs 26.

Because of that `electric-pair-post-self-insert-function', called by 
`post-self-insert-hook', does not skip the insertion of the right angle 
bracket character.

The following snippet evaluates to "60" on both Emacs 26 and 27, so I 
don't understand where the "Unbalanced parentheses" error could come from.

----
<div>
(progn
  (electric-pair-mode 1)
  (html-mode)
  (let ((table (syntax-table))) (electric-pair--with-uncached-syntax (table) 
(matching-paren (char-before (scan-sexps 0 1))))))
----

Gregory