[Webkit-unassigned] [Bug 131839] The OS/2 USE_TYPO_METRICS fsSelection flag is not taken into account

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=131839

--- Comment #3 from Frédéric Wang (:fredw) <fred.wang at free.fr> ---
Some update on this, now that the situation is clearer.

1) Sfnt fonts have several vertical metrics:

  a) The "typo" metrics from the OS/2 table
  b) The "Windows" metrics from the OS/2 table
  c) The metrics from the hhea table

Since math fonts contain tall glyphs (e.g. integrals) the metrics from b) are often much larger than those from a). Depending on the fonts, the metrics from c) may also be larger than those from a).

2) The OS/2 table contains a USE_TYPO_METRICS flag to explicitly ask to use the metrics from a). At the time of writing,

  i) This flag is now taken into account on all platforms in Gecko.
  ii) STIX 1.1 does not set this flag. We'll have to wait version 2 which is currently in development.
  iii) All the other fonts with a MATH table sets the flag.

We should probably do the same for WebKit for all the platforms supported. I can at least verify that the flag is not taken into account on Linux (using epiphany) with the following testcase:

http://tests.mathml-association.org/mathml/relations/text-and-math/use-typo-metrics-1.html

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