in TextBuilderWithHighlight.build, check when we encounter a \n if we would be past the last line, and return the acc if so, or else continue looping

fcore/text-builder/text-builder-with-highlight.sml

@@ -236,7 +236,11 @@ struct
                     Utils.makeCursor (posX, posY, env) :: acc
                   else
                     acc
+                val nextLineNumber = lineNumber + 1
               in
+                if nextLineNumber > #lastLineNumber env then
+                  acc
+                else
                   build
                     ( pos + 1
                     , str
@@ -246,7 +250,7 @@ struct
                     , #startX env
                     , posY + TC.ySpace
                     , 0
-                  , lineNumber + 1
+                    , nextLineNumber
                     , absIdx + 1
                     , cursorIdx
                     , env