Markdown reader: fix small super/subscript issue.

Superscripts and subscripts cannot contain spaces,
but newlines were previously allowed (unintentionally).
This led to bad interactions in some cases with footnotes.
E.g.

```
foo^[note]
bar^[note]
```

With this change newlines are also not allowed inside
super/subscripts.

Closes #5878.

MANUAL.txt

@@ -4042,12 +4042,13 @@ text by `~` characters.  Thus, for example,
 
     H~2~O is a liquid.  2^10^ is 1024.
 
-If the superscripted or subscripted text contains spaces, these spaces
-must be escaped with backslashes.  (This is to prevent accidental
-superscripting and subscripting through the ordinary use of `~` and `^`.)
-Thus, if you want the letter P with 'a cat' in subscripts, use
-`P~a\ cat~`, not `P~a cat~`.
-
+The text between `^...^` or `~...~` may not contain spaces or
+newlines.  If the superscripted or subscripted text contains
+spaces, these spaces must be escaped with backslashes.  (This is
+to prevent accidental superscripting and subscripting through
+the ordinary use of `~` and `^`, and also bad interactions with
+footnotes.) Thus, if you want the letter P with 'a cat' in
+subscripts, use `P~a\ cat~`, not `P~a cat~`.
 
 ### Verbatim ###
 

src/Text/Pandoc/Readers/Markdown.hs

@@ -1700,13 +1700,17 @@ superscript :: PandocMonad m => MarkdownParser m (F Inlines)
 superscript = fmap B.superscript <$> try (do
   guardEnabled Ext_superscript
   char '^'
-  mconcat <$> many1Till (notFollowedBy spaceChar >> inline) (char '^'))
+  mconcat <$> many1Till (do notFollowedBy spaceChar
+                            notFollowedBy newline
+                            inline) (char '^'))
 
 subscript :: PandocMonad m => MarkdownParser m (F Inlines)
 subscript = fmap B.subscript <$> try (do
   guardEnabled Ext_subscript
   char '~'
-  mconcat <$> many1Till (notFollowedBy spaceChar >> inline) (char '~'))
+  mconcat <$> many1Till (do notFollowedBy spaceChar
+                            notFollowedBy newline
+                            inline) (char '~'))
 
 whitespace :: PandocMonad m => MarkdownParser m (F Inlines)
 whitespace = spaceChar >> return <$> (lb <|> regsp) <?> "whitespace"

test/command/5878.md

@@ -0,0 +1,7 @@
+```
+% pandoc -t native
+Zozime^[],
+Synésius^[]
+^D
+[Para [Str "Zozime",Note [Para []],Str ",",SoftBreak,Str "Syn\233sius",Note [Para []]]]
+```