fix exception when the buffer is empty

fcore/text-builder/normal-mode-text-builder.sml

@@ -19,53 +19,70 @@ struct
     , acc
     ) =
     let
-      val {rightStrings, rightLines, line = curLine, idx = curIdx, ...} = buffer
+      val
+        { rightStrings
+        , rightLines
+        , line = curLine
+        , idx = curIdx
+        , textLength
+        , ...
+        } = buffer
+
+      val env = Utils.initEnv
+        ( 5
+        , 5
+        , windowWidth
+        , windowHeight
+        , floatWindowWidth
+        , floatWindowHeight
+        , searchList
+        , String.size searchString
+        , visualScrollColumn
+        , startLine
+        )
+      val {startX, startY, ...} = env
     in
-      case (rightStrings, rightLines) of
-        (shd :: stl, lhd :: ltl) =>
-          let
-            (* get relative index of line to start building from *)
-            val strPos =
-              Utils.getRelativeLineStartFromRightHead (startLine, curLine, lhd)
-            (* get absolute idx of line *)
-            val absIdx = curIdx + strPos
+      if textLength = 1 then
+        (* empty string, so there is nothing we can draw 
+         * except a cursor at the line start.
+         * An empty string is usually thought of to have a length of 0 
+         * and this is true, but we always have a \n at the end of the buffer
+         * to respect Unix-style file endings, which we always uphold. 
+         * So, for us, an empty string has a length of 1. *)
+        [Utils.makeCursor (startX, startY, env)]
+      else
+        case (rightStrings, rightLines) of
+          (shd :: stl, lhd :: ltl) =>
+            let
+              (* get relative index of line to start building from *)
+              val strPos =
+                Utils.getRelativeLineStartFromRightHead
+                  (startLine, curLine, lhd)
+              (* get absolute idx of line *)
+              val absIdx = curIdx + strPos
 
-            val searchPos = BinSearch.equalOrMore (absIdx, searchList)
-            val searchPos =
-              if searchPos = ~1 then Vector.length searchList else searchPos
-
-            val env = Utils.initEnv
-              ( 5
-              , 5
-              , windowWidth
-              , windowHeight
-              , floatWindowWidth
-              , floatWindowHeight
-              , searchList
-              , String.size searchString
-              , visualScrollColumn
-              , startLine
-              )
-            val {startX, startY, ...} = env
-          in
-            TextBuilderWithHighlight.build
-              ( strPos
-              , shd
-              , stl
-              , lhd
-              , ltl
-              , startX
-              , startY
-              , 0
-              , startLine
-              , absIdx
-              , cursorPos
-              , env
-              , acc
-              , searchPos
-              )
-          end
-      | (_, _) => acc
+              val searchPos = BinSearch.equalOrMore (absIdx, searchList)
+              val searchPos =
+                if searchPos = ~1 then Vector.length searchList else searchPos
+            in
+              TextBuilderWithHighlight.build
+                ( strPos
+                , shd
+                , stl
+                , lhd
+                , ltl
+                , startX
+                , startY
+                , 0
+                , startLine
+                , absIdx
+                , cursorPos
+                , env
+                , acc
+                , searchPos
+                )
+            end
+        | (_, _) => acc
     end
 
   fun buildWithExisting