1063282 - investigate why system font fallback occurs so often for Arabic text runs

Status: NEW

(Core :: Graphics: Text, defect)

Description

[John Daggett (:jtd)]

Users of the Arabic locale represent less than 1% of total ADUs for August, yet for a similar period over 30% of system fallback cases were for Arabic, based on telemetry data.

System font fallback script (i.e. script of fallback character) for Release 31
http://mzl.la/Z9dc9N

Highlight data (% of total fallbacks per script when >1%):

14.87%	common
31.17%	arabic
1.64%	bengali
3.10%	cyrillic
2.24%	han
35.02%	latin
1.18%	myanmar
4.69%	thai

Script codes used
http://mxr.mozilla.org/mozilla-central/source/intl/unicharutil/util/nsUnicodeScriptCodes.h

Ex: Latin == 25, add one to compare to telemetry data so look for '26' in the first column.

The system fallback script only occurs after pref fonts have been tried. For well-supported scripts like Arabic, system font fallback generally shouldn't occur that often. I'm guessing we need to confirm that Arabic pref fonts are picked up for Arabic text runs.

Comment 1

[David Baron :dbaron: (⌚️UTC-4, no longer working on Mozilla)]

Is it also worth confirming that the default Arabic pref fonts are fonts that people actually have?

Comment 2

[John Daggett (:jtd)]

(In reply to David Baron [:dbaron] (UTC-5) (needinfo? for questions) from comment #1)
> Is it also worth confirming that the default Arabic pref fonts are fonts
> that people actually have?

Yes, I think I need to look through the telemetry data and dig into which OS this is occurring on. XP? Win7? Might even make sense to try and track down which font is used instead of the pref font for Arabic in these fallback cases by putting in additional telemetry related to Arabic specifically.

Comment 3

[Christian Sonne [:cers]]

I think I came upon an example of this happening (at least on OS X) on Reddit: https://www.reddit.com/r/css/comments/4r9otc/help_fontface_not_working_with_firefox_tried/

I don't know if the page will remain live, but at the moment it can be seen in effect here: http://168.235.88.189/splash.html (I didn't want to attach a copy here, as the font in question isn't free)

I've determined that certain characters in the text makes Firefox use a fallback system font instead - in fact, I think most of them in this example.