fix(math): Add math.font.script.feature setting, defaulting to ssty

Author: Omikhleia

packages/math/base-elements.lua

@@ -947,6 +947,17 @@ end
 
 function elements.text:shape ()
    self.font.size = self.font.size * self:getScaleDown()
+   if isScriptMode(self.mode) then
+      local scriptFeature = SILE.settings:get("math.font.script.feature")
+      if scriptFeature then
+         self.font.features = ("+%s=1"):format(scriptFeature)
+      end
+   elseif isScriptScriptMode(self.mode) then
+      local scriptFeature = SILE.settings:get("math.font.script.feature")
+      if scriptFeature then
+         self.font.features = ("+%s=2"):format(scriptFeature)
+      end
+   end
    local face = SILE.font.cache(self.font, SILE.shaper.getFace)
    local mathMetrics = self:getMathMetrics()
    local glyphs = SILE.shaper:shapeToken(self.text, self.font)

packages/math/init.lua

@@ -35,6 +35,12 @@ function package.declareSettings (_)
       type = "integer",
       default = 400,
    })
+   SILE.settings:declare({
+      parameter = "math.font.script.feature",
+      type = "string or nil",
+      default = "ssty",
+      help = "OpenType feature for the math script-style alternates (esp. primes), theoretically ssty",
+   })
    SILE.settings:declare({
       parameter = "math.font.filename",
       type = "string",
@@ -138,6 +144,10 @@ By default, this package uses Libertinus Math, so it will fail if Libertinus Mat
 Another font may be specified via the setting \autodoc:setting{math.font.family}.
 If required, you can set the font style and weight via \autodoc:setting{math.font.style} and \autodoc:setting{math.font.weight}.
 The font size can be set via \autodoc:setting{math.font.size}.
+The \autodoc:setting{math.font.script.feature} setting can be used to specify OpenType features for the math font, which are applied to the smaller script styles.
+It defaults to \code{ssty} (script style alternates), notably to ensure that some symbols such as the prime, double prime, etc. are displayed correctly.
+The default setting applies to Libertinus Math and well-designed math fonts, but some fonts may require different features.
+(The STIX Two Math font has a stylitic set \code{ss04} from primes only, but also supports, according to its documentation, \code{ssty}, which provides other optical adjustments.)
 
 \paragraph{MathML}
 The first way to typeset math formulas is to enter them in the MathML format.