fix(math): Avoid page breaks before display math equations

See sile-typesetter#2160.
This is a conservative workaround.
Introducing the predisplay/postdisplay penalties is the right thing to
do, but a more general solution to the full issue requires a more subtle
handling of in-paragraph display math equations, which is not considered
here.

packages/math/init.lua

@@ -56,6 +56,31 @@ function package.declareSettings (_)
       type = "VGlue",
       default = SILE.types.node.vglue("2ex plus 1pt"),
    })
+
+  -- Penalties for breaking before and after a display math formula
+  -- See TeX's \predisplaypenalty and \postdisplaypenalty
+  SILE.settings:declare({
+     parameter = "math.predisplaypenalty",
+     type = "integer",
+     default = 10000, -- strict no break by default as in (La)TeX
+     help = "Penalty for breaking before a display math formula",
+  })
+  SILE.settings:declare({
+     parameter = "math.postdisplaypenalty",
+     type = "integer",
+     -- (La)TeX's default is 0 (a normal line break penalty allowing a break
+     -- after a display math formula)
+     -- See https://github.com/sile-typesetter/sile/issues/2160
+     --    And see implementation in handleMath(): we are not yeat doing the right
+     --    things with respect to paragraphing, so setting a lower value for now
+     --    to ease breaking after a display math formula rather than before
+     --    when the formula is in the middle of a paragraph.
+     --    (In TeX, these penalties would apply in horizontal mode, with a display
+     --    math formula being a horizontal full-width box, our implementation
+     --    currently use them as vertical penalties).
+     default = -50,
+     help = "Penalty for breaking after a display math formula",
+  })
 end
 
 function package:registerCommands ()

packages/math/typesetter.lua

@@ -211,8 +211,15 @@ local function handleMath (_, mbox, options)
    mbox:shapeTree()
 
    if mode == "display" then
-      SILE.typesetter:endline()
+      -- See https://github.com/sile-typesetter/sile/issues/2160
+      --    We are not excactly doing the right things here with respect to
+      --    paragraphing expectations.
+      -- The vertical penalty will flush the previous paragraph, if any.
+      SILE.call("penalty", { penalty = SILE.settings:get("math.predisplaypenalty"), vertical = true })
       SILE.typesetter:pushExplicitVglue(SILE.settings:get("math.displayskip"))
+      -- Repeating the penalty after the skip does not hurt but should not be
+      -- necessary if our page builder did its stuff correctly.
+      SILE.call("penalty", { penalty = SILE.settings:get("math.predisplaypenalty"), vertical = true })
       SILE.settings:temporarily(function ()
          -- Center the equation in the space available up to the counter (if any),
          -- respecting the fixed part of the left and right skips.
@@ -233,9 +240,14 @@ local function handleMath (_, mbox, options)
          elseif options.number then
             SILE.call("math:numberingstyle", options)
          end
-         SILE.typesetter:endline()
+         -- The vertical penalty will flush the equation.
+         -- It must be done in the temporary settings block, because these have
+         -- to apply as line boxes are being built.
+         SILE.call("penalty", { penalty = SILE.settings:get("math.postdisplaypenalty"), vertical = true })
       end)
       SILE.typesetter:pushExplicitVglue(SILE.settings:get("math.displayskip"))
+      -- Repeating: Same remark as for the predisplay penalty above.
+       SILE.call("penalty", { penalty = SILE.settings:get("math.postdisplaypenalty"), vertical = true })
    else
       SILE.typesetter:pushHorizontal(mbox)
    end