Ignore weird par nodes in LMTX

Fixes #45, for real this time. (73445ca fixed lua-widow-control with
LuaMetaTeX in general, but it didn't actually fix _this_ issue. Whoops.)

source/lua-widow-control.lua

@@ -100,6 +100,7 @@ local hlist_id = id_from_name("hlist")
 local line_subid = 1
 local mark_id = id_from_name("mark")
 local par_id = id_from_name("par") or id_from_name("local_par")
+local par_subids = table.swapped(node.subtypes("par") or {})
 local parfill_subids = {
     parfillleftskip = 17,
     parfillrightskip = 16,
@@ -560,13 +561,16 @@ end
 --- @param head node The pre-broken paragraph
 --- @return node head The unmodified `head` argument
 function lwc.save_paragraphs(head)
-    if (head.id ~= par_id and context) or -- Make sure that `head` is a paragraph
+    if (context and head.id ~= par_id) or -- Make sure that `head` is a paragraph
         status.output_active or -- Don't run during the output routine
-        tex.nest.ptr > 1 -- Don't run inside boxes
+        tex.nest.ptr > 1 or -- Don't run inside boxes
+       (lmtx and not ( -- Check for weird `par` nodes
+            head.subtype == par_subids.vmodepar or
+            head.subtype == par_subids.hmodepar
+       ))
     then
         return head
     end
-
     -- Prevent the "underfull hbox" warnings when we store a potential paragraph
     local renable_box_warnings
     if (context or optex) or

tests/context/description.lvtext

@@ -0,0 +1,39 @@
+% lua-widow-control
+% https://github.com/gucci-on-fleek/lua-widow-control
+% SPDX-License-Identifier: MPL-2.0+
+% SPDX-FileCopyrightText: 2024 Max Chernoff
+
+% See https://github.com/gucci-on-fleek/lua-widow-control/issues/45
+
+\usemodule[lua-widow-control]
+\setuplwc[showcolours=start]
+
+\setuppapersize[A6]
+
+\setupindenting[yes, 3em]
+
+\definefontfeature[default][default][expansion=quality,protrusion=quality]
+
+\usetypescript[modern-base]
+\setupbodyfont[reset,modern]
+
+\setupalign[hz, hanging, verytolerant]
+
+\definedescription[concept]
+
+\setupdescription[concept][
+    indenting=yes,
+    width=fit,
+]
+
+\startdocument
+    \samplefile{knuth}
+
+    \dorecurse{3}{
+        \startconcept{Concept}
+            \dorecurse{8}{
+                Filler \dorecurse{15}{ filler}.\par
+            }
+        \stopconcept
+    }
+\stopdocument