bug#20984: 25.0.50; Combining accents don't display properly in certain fonts

[Artur Malabarba]

2015-07-05 17:33 GMT+01:00 Eli Zaretskii <eliz@gnu.org>:
>> Date: Sun, 5 Jul 2015 17:24:32 +0100
>> From: Artur Malabarba <bruce.connor.am@gmail.com>
>>
>> Start emacs -Q then evaluate the following in the *scratch* (assuming
>> you have this font installed)
>>
>> (set-face-attribute 'default nil :font "SourceCodePro Medium")
>> (insert (format "a%c" 768))
>>
>> The result should look like "à", but instead, it looks like "a`" where
>> the letter "a" is actually distorted and shifted up (image attached).
>> The same happens with other combining symbols.
>
> Do both glyphs come from the same font, according to "C-u C-x ="?  (I
> don't have that font installed to try that myself.)
>
> Emacs can only compose characters if their glyphs are covered by the
> same font.

Yes. Investigating the weird combination yields this on the description buffer:

Composed with the following character(s) "̀" using this font:
  xft:-adobe-Source Code Pro-normal-normal-normal-*-13-*-*-*-m-0-iso10646-1
by these glyphs:
  [0 1 0 626 8 1 6 8 -3 nil]
  [0 1 0 734 0 1 5 11 -9 [0 0 0]]

And investigating the characters individually yields the same font for both

    xft:-adobe-Source Code
Pro-normal-normal-normal-*-13-*-*-*-m-0-iso10646-1 (#x1C)
    xft:-adobe-Source Code
Pro-normal-normal-normal-*-13-*-*-*-m-0-iso10646-1 (#x2DD)

Should I paste the full *help* buffer here?