fix(math): Stretchy symbols should work on large content (workaround)

The current stretchy "reshaping" picks the closest symbols in the math
glyph variants, but when none exist or they are still not close enough
(esp. far too small), OpenTypeand MathML Core say we should rely on
complex glyph assembly rules from glyph parts.
We don't have that yet: Here we fallback on re-scaling the glyph.
It might not look always good, but at least it's stretched as it
should around the content. It's a dirty compromise until something
better can be done.

Author: Omikhleia

packages/math/base-elements.lua

@@ -1035,12 +1035,26 @@ function elements.text:_stretchyReshape (target, direction)
    return false
 end
 
-function elements.text:_vertStretchyReshape(depth, height)
-   return self:_stretchyReshape(depth + height, true)
+function elements.text:_vertStretchyReshape (depth, height)
+   local hasStretched = self:_stretchyReshape(depth + height, true)
+   if hasStretched then
+      -- HACK: see output routine
+      self.vertExpectedSz = height + depth
+      self.vertScalingRatio = (depth + height):tonumber() / (self.height:tonumber() + self.depth:tonumber())
+      self.height = height
+      self.depth = depth
+   end
+   return hasStretched
 end
 
-function elements.text:_horizStretchyReshape(width)
-   return self:_stretchyReshape(width, false)
+function elements.text:_horizStretchyReshape (width)
+   local hasStretched = self:_stretchyReshape(width, false)
+   if hasStretched then
+      -- HACK: see output routine
+      self.horizScalingRatio = width:tonumber() / self.width:tonumber()
+      self.width = width
+   end
+   return hasStretched
 end
 
 function elements.text:output (x, y, line)
@@ -1058,7 +1072,21 @@ function elements.text:output (x, y, line)
    -- There should be no stretch or shrink on the width of a text
    -- element.
    local width = self.width.length
-   SILE.outputter:drawHbox(self.value, width)
+   -- HACK: For stretchy operators, MathML Core and OpenType define how to build large glyphs
+   -- from an assembly of smaller ones. It's fairly complex and idealistic...
+   -- Anyhow, we do not have that yet, so we just stretch the glyph artificially.
+   -- There are cases where this will not look very good.
+   -- Call that a compromise, so that long vectors or large matrices look "decent" without assembly.
+   if SILE.outputter.scaleFn and (self.horizScalingRatio or self.vertScalingRatio) then
+      local xratio = self.horizScalingRatio or 1
+      local yratio = self.vertScalingRatio or 1
+      SU.debug("math", "fake glyph stretch: xratio =", xratio, "yratio =", yratio)
+      SILE.outputter:scaleFn(x, y, xratio, yratio, function ()
+         SILE.outputter:drawHbox(self.value, width)
+      end)
+   else
+      SILE.outputter:drawHbox(self.value, width)
+   end
 end
 
 elements.fraction = pl.class(elements.mbox)