Subject: Re: bug#26396: 25.1; char-displayable-p on a latin1 tty

From: Paul Eggert <[email-masked]>
Date: Tue, 18 Apr 2017 10:49:37 -0700

On 04/17/2017 11:32 AM, Eli Zaretskii wrote:
> Can you show an example of a character displayed in different forms
> depending on buffer contents?  I'd like to look what the code does and why.

In master on the Linux console in non-UTF-8 mode and with a unibyte 
en_US locale, if I run 'emacs -Q' and type 'C-x 8 RET 100 RET C-x 8 RET 
200 RET' the screen looks like this:

\u0100\u0200

If I then type 'C-x 8 RET 300 RET', the '\u0200' magically changes to 
'?' and another '?' is appended, so that the screen then looks like this:

\u0100??

Presumably this is some sort of combining-character thing. However, if 
the intent is to present a combined character, shouldn't the character 
be displayed as a single '?', to better mimic the single glyph you'd see 
on an X display?

By the way, the '?'s look like ordinary question marks; they are not 
highlighted, as the \u0100 is. Shouldn't they be highlighted somehow? 
And while I have your ear, why is U+0700 SYRIAC END OF PARAGRAPH 
displayed as an ordinary '?' while U+0500 CYRILLIC CAPITAL LETTER KOMI 
DE is displayed as a highlighted '\u0500'?