in BinSearch.equalOrMore and BinSearch.equalOrLess, return ~1 if no item is found, for the sake of less ambiguity in usage. Functions that call these have also been adapted.

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

@@ -29,7 +29,10 @@ struct
               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
               ( windowWidth

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

@@ -60,7 +60,7 @@ struct
       let
         val searchPos = BinSearch.equalOrMore (pos + 1, #searchList env)
       in
-        if searchPos = Vector.length line then
+        if searchPos = ~1 then
           (* next line is not in this node *)
           let
             val absIdx = absIdx - pos

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

@@ -75,7 +75,7 @@ struct
       let
         val searchPos = BinSearch.equalOrMore (pos + 1, #searchList env)
       in
-        if searchPos = Vector.length line then
+        if searchPos = ~1 then
           (* next line is not in this node *)
           let
             val absIdx = absIdx - pos