[tor-qa] Font debugging page (TBB 5.0a4)

[Arthur D. Edelstein]

On Thu, Aug 6, 2015 at 11:09 AM, Arthur D. Edelstein
<arthuredelstein at gmail.com> wrote:
> On Thu, Aug 6, 2015 at 9:41 AM, David Fifield <david at bamsoftware.com> wrote:
>> On Thu, Aug 06, 2015 at 01:36:57PM +0200, Moritz Bartl wrote:
>>> On 08/06/2015 11:43 AM, David Fifield wrote:
>>> > Oh, very interesting. It seems the difference is only in one code point,
>>> > in the "cursive" and "fantasy" styles (last two columns): [...]
>>> > Could you make screenshots of the two renderings?
>>>
>>> Attached. I don't see any difference between default and cursive.
>>
>> That's very interesting. In just that one code point, your computer
>> chooses a sans-serif font for the "fantasy" and "cursive" styles, while
>> mine chooses a serif font. Probably we would find more examples if we
>> tested more of Unicode.
>>
>> I would not have expected a difference like this. It does not come down
>> to small rendering differences; the browser is simply choosing a
>> different font. Perhaps there is something non-deterministic about how
>> Firefox (or Fontconfig) loads its font files. Like, maybe it iterates a
>> directory in inode order and takes the first match when there is a tie.
>
> If you add "Noto Serif Armenian" or "Noto Sans Armenian" to the lists
> in the prefs "font.name-list.fantasy.x-unicode" and
> "font.name-list.cursive.x-unicode" (these can be edited in
> about:config) does that determine the font chosen for that code point
> and allow you to get a checksum match? If so, it seems we should
> include as many fonts as possible in each "name-list" to ensure the
> font priority is consistent across platforms.

I'm finding that if I modify "font.name-list.fantasy.x-western" to
include "Noto Sans Armenian" or "Noto Sans Serif" I control which of
the two fonts is chosen. Unfortunately, if I add the attribute
`lang="jp"` to the HTML span element enclosing the glyph (using the
Firefox dev tools), then the font chosen is determined instead by
"font.name-list.fantasy.x-unicode".

So it looks like we need to provide the order of fonts to every
"font.name-list.*" pref whenever there might be more than one font
covering the same code point. In the currently bundled collection,
there are only a few fonts with overlapping code point coverage, so it
shouldn't be too bad, I hope.