Hi,

When exporting org-mode document to plain text (either ascii/unicode/utf-8)
with `org-export-dispatch', Emacs inserts lines under headlines, inline
tasks, table rows and titles of the document, TOC, list of listings, list of
tables and footnotes.  The problem is it inserts too long (double width) line
when the line character has a width of 2.

Those lines are made of 3 types of characters below (in ox-ascii.el):
1) org-ascii-underline
2) (if (eq (plist-get info :ascii-charset) 'utf-8) ?─ ?_)
3) (if utf8p ?━ ?_)

In case of 1), it correctly takes account of the case in which the character
has a width of 2 in `org-ascii--build-title', by dividing the line width by
`(char-width under-char)' (line 700-701), maybe because the character is user
configurable and its width in unknown.  However, in case of 2) and
3), maybe because the characters is embedded in the code, it looks like only
considering the character always has a width of 1.  But the reality is
character ?─ or ?━ can have a width of 2 in the screen displayed with some
fonts (ex. "Noto Sans Mono CJK JP"), and in that case the line width gets
doubled of the expected width.

Attached one is a potential patch.  The basic concepts are:

a) Do the same in case of 2) and 3) as in case of 1)
   (dividing the line width by `(char-width under-char)',
    assuming `char-width-table' is correctly set)
    
b) Prefer the longer line width if the width is odd, even in case of 1)
   (adding `(1- (char-width under-char))' to dividend,
    just because it should be more beautiful ;-) )

Regards,
---
Shingo Tanaka