don't index into line vector when getting line number from index; instead, just get how many previous-lines there are in the vector (if any) and add it to the line number of the current node. We do this because we are trying to get the line number of a particular index; accessing the line vector's contents (which are string indices) is not necessary nor helpful for that, but getting the relative line number is helpful for that

src/line_gap.sml

@@ -2666,8 +2666,7 @@ struct
                 val relativeIdx = findIdx - prevIdx
                 val relativeLine = binSearch (relativeIdx, lhd)
               in
-                if relativeLine < 0 then prevLine
-                else Vector.sub (lhd, relativeLine) + prevLine
+                if relativeLine < 0 then prevLine else prevLine + relativeLine
               end
           else
             let val prevLine = curLine - Vector.length lhd
@@ -2691,10 +2690,8 @@ struct
               let
                 val relativeIdx = findIdx - curIdx
                 val relativeLine = binSearch (relativeIdx, lhd)
-                val lineOffset =
-                  if relativeLine < 0 then 0 else Vector.sub (lhd, relativeLine)
               in
-                curLine + lineOffset
+                if relativeLine < 0 then curLine else curLine + relativeLine
               end
           else
             let val nextLine = curLine + Vector.length lhd