feat(math): Support MathML mpadded and refactor TeX-like phantoms

Nothing really complex with mpadded.. Wait some ambiguity if dimensions
are in a font-relative unit (see in-code comment), but let's move
forward anyway -- and on the same occasion, remove the non-standard
approach previously used for (La)TeX's phantom, hphantom and vphantom.

Author: Omikhleia

packages/math/base-elements.lua

@@ -468,7 +468,7 @@ function elements.stackbox.output (_, _, _, _) end
 elements.phantom = pl.class(elements.stackbox) -- inherit from stackbox
 elements.phantom._type = "Phantom"
 
-function elements.phantom:_init (children, special)
+function elements.phantom:_init (children)
    -- MathML core 3.3.7:
    -- "Its layout algorithm is the same as the mrow element".
    -- Also not the MathML states that <mphantom> is sort of legacy, "implemented
@@ -477,22 +477,6 @@ function elements.phantom:_init (children, special)
    -- The thing is that we don't have CSS in SILE, so supporting <mphantom> is
    -- a must.
    elements.stackbox._init(self, "H", children)
-   self.special = special
-end
-
-function elements.phantom:shape ()
-   elements.stackbox.shape(self)
-   -- From https://latexref.xyz:
-   -- "The \vphantom variant produces an invisible box with the same vertical size
-   -- as subformula, the same height and depth, but having zero width.
-   -- And \hphantom makes a box with the same width as subformula but
-   -- with zero height and depth."
-   if self.special == "v" then
-      self.width = SILE.types.length()
-   elseif self.special == "h" then
-      self.height = SILE.types.length()
-      self.depth = SILE.types.length()
-   end
 end
 
 function elements.phantom:output (_, _, _)
@@ -1454,6 +1438,50 @@ function elements.sqrt:output (x, y, line)
    )
 end
 
+elements.padded = pl.class(elements.mbox)
+elements.padded._type = "Padded"
+
+function elements.padded:__tostring ()
+   return self._type .. "(" .. tostring(self.impadded) .. ")"
+end
+
+function elements.padded:_init (attributes, impadded)
+   elements.mbox._init(self)
+   self.impadded = impadded
+   self.attributes = attributes or {}
+   table.insert(self.children, impadded)
+end
+
+function elements.padded:styleChildren ()
+   self.impadded.mode = self.mode
+end
+
+function elements.padded:shape ()
+   -- TODO MathML allows percentages font-relative units (em, ex) for padding
+   -- But our units work with font.size, not math.font.size (possibly adjusted by scaleDown)
+   -- so the expectations might not be met.
+   local width = self.attributes.width and SU.cast("measurement", self.attributes.width)
+   local height = self.attributes.height and SU.cast("measurement", self.attributes.height)
+   local depth = self.attributes.depth and SU.cast("measurement", self.attributes.depth)
+   local lspace = self.attributes.lspace and SU.cast("measurement", self.attributes.lspace)
+   local voffset = self.attributes.voffset and SU.cast("measurement", self.attributes.voffset)
+   -- Clamping for width, height, depth, lspace
+   width = width and (width:tonumber() > 0 and width or SILE.types.measurement())
+   height = height and (height:tonumber() > 0 and height or SILE.types.measurement())
+   depth = depth and (depth:tonumber() > 0 and depth or SILE.types.measurement())
+   lspace = lspace and (lspace:tonumber() > 0 and lspace or SILE.types.measurement())
+   -- No clamping for voffset
+   voffset = voffset or SILE.types.measurement(0)
+   -- Compute the dimensions
+   self.width = width and SILE.types.length(width) or self.impadded.width
+   self.height = height and SILE.types.length(height) or self.impadded.height
+   self.depth = depth and SILE.types.length(depth) or self.impadded.depth
+   self.impadded.relX = lspace and SILE.types.length(lspace) or SILE.types.length()
+   self.impadded.relY = voffset and SILE.types.length(voffset):negate() or SILE.types.length()
+end
+
+function elements.padded.output (_, _, _, _) end
+
 elements.mathMode = mathMode
 elements.atomType = atomType
 elements.symbolDefaults = symbolDefaults

packages/math/texlike.lua

@@ -656,8 +656,9 @@ compileToMathML(
 
   % Phantom commands from TeX/LaTeX
   \def{phantom}{\mphantom{#1}}
-  \def{hphantom}{\mphantom[special=h]{#1}}
-  \def{vphantom}{\mphantom[special=v]{#1}}
+  \def{hphantom}{\mpadded[height=0, depth=0]{\mphantom{#1}}}
+  \def{vphantom}{\mpadded[width=0]{\mphantom{#1}}}
+  %\mphantom[special=v]{#1}}}
 ]==],
    })
 )

packages/math/typesetter.lua

@@ -177,6 +177,10 @@ function ConvertMathML (_, content)
       -- It's an mrow, but with some style attributes that we ignore.
       SU.warn("MathML mstyle is not fully supported yet")
       return b.stackbox("H", convertChildren(content))
+   elseif content.command == "mpadded" then
+      -- MathML Core 3.3.6.1: The <mpadded> element generates an anonymous <mrow> box
+      -- called the "impadded inner box"
+      return b.padded(content.options, b.stackbox("H", convertChildren(content)))
    else
       SU.error("Unknown math command " .. content.command)
    end