handle edge case in line_gap.sml when deleting to the left: we sometimes need to delete to the end of the string, so add a branch handling that case

src/line_gap.sml

@@ -1971,6 +1971,55 @@ struct
                     )
                 end
               else if prevIdx < start then
+                if finish >= curIdx then
+                  (* delete from end of string *)
+                  let
+                    val oldNodeTextLength = String.size leftStringsHd
+                    val oldNodeLineLength = Vector.length leftLinesHd
+
+                    val sub1Length = start - prevIdx
+                    val sub1 = String.substring (leftStringsHd, 0, sub1Length)
+                    val sub1Lines =
+                      if Vector.length leftLinesHd > 0 then
+                        let
+                          val midpoint = binSearch
+                            (String.size sub1 - 1, leftLinesHd)
+                        in
+                          if midpoint >= 0 then
+                            let
+                              val slice = VectorSlice.slice
+                                (leftLinesHd, 0, SOME (midpoint + 1))
+                            in
+                              VectorSlice.vector slice
+                            end
+                          else
+                            Vector.fromList []
+                        end
+                      else
+                        leftLinesHd
+                    val newNodeTextLength = String.size sub1
+                    val textLengthDifference =
+                      oldNodeTextLength - newNodeTextLength
+                    val textLength = textLength - textLengthDifference
+
+                    val newNodeLineLength = Vector.length sub1Lines
+                    val lineLengthDifference =
+                      oldNodeLineLength - newNodeLineLength
+                    val lineLength = lineLength - lineLengthDifference
+                  in
+                    { idx = prevIdx + String.size sub1
+                    , line =
+                        (curLine - Vector.length leftLinesHd)
+                        + Vector.length sub1Lines
+                    , leftStrings = sub1 :: leftStringsTl
+                    , leftLines = sub1Lines :: leftLinesTl
+                    , rightStrings = rightStrings
+                    , rightLines = rightLines
+                    , textLength = textLength
+                    , lineLength = lineLength
+                    }
+                  end
+                else
                   (* We want to delete in the middle of leftStringsHd. 
                    * We also have to delete in the middle of leftLinesHd in order to
                    * do this. *)