bug#20628: 25.0.50; Incorrect line height for some fonts

[Clément Pit--Claudel]

On 05/29/2015 06:15 AM, Eli Zaretskii wrote:
>> From: Oleh Krehel <ohwoeowho@gmail.com>
>> Cc: Eli Zaretskii <eliz@gnu.org>,  wl@gnu.org,  20628@debbugs.gnu.org
>> Date: Fri, 29 May 2015 11:30:55 +0200
>>
>> Clément Pit--Claudel <clement.pitclaudel@live.com> writes:
>>
>>> The patch partially solves the problem for me, but I noticed a few
>>> problems after running (set-frame-font "Latin Modern Math")
>>> * When the cursor is at the end of the file, on an empty line, it has a 
>>> very height.
>                                                                             
> ^^^^^^^^^^^
> 
> "Very WHAT height"? "very small" or "very large"?

Woops, sorry. Very large height.

>>> * The fix seems to only apply to certain characters. The line that I
>>> mentioned in my original email, in particular, is still very tall. In
>>> other words, when trying the following three lines in a Latin Modern
>>> Math buffer, the last line is much too tall:
>>>
>>> 𝓝  ;; This is still very tall
> 
> I don't see this on my system.  Here, it makes the line slightly
> higher, and that's all. Perhaps you have a different version of the
> font installed.  But in any case, if that character still shows up as
> too large (a screenshot would be nice), and Emacs uses the Latin
> Modern Math font to display it (make sure with "C-u C-x ="), then
> there's nothing that can be done about that, since it means the
> metrics of the glyph itself, as reported by the font/font driver, are
> screwed.  If you want to make sure that's the reason, put a breakpoint
> on line 26425 of xdisp.c, after the call to get_per_char_metric, as
> shown below:
> 
>         if (get_char_glyph_code (it->char_to_display, font, &char2b))
>           {
>             pcm = get_per_char_metric (font, &char2b);
>             if (pcm->width == 0    <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
>                 && pcm->rbearing == 0 && pcm->lbearing == 0)
>               pcm = NULL;
>           }
> 
> and see what ascent and descent values are reported for that character
> in pcm->ascent and pcm->descent.

Thanks for the detailed explanation! You were right, the height is fine if the 
character itself is inserted. The problem is due to prettify-symbols-mode.

Thanks for your work on this, it's very cool! This is a bug that had annoyed me 
4 or 5 years ago when I first used Emacs, but at that time I didn't really know 
how to report bugs.