improvements to TextWindow.isCursorVisible (no need to do binary search; in fact, that leads to an exception, because we are meant to use Vector.sub to access the relative line instead)

fcore/text-window.sml

@@ -319,9 +319,13 @@ struct
 
   fun startIsCursorVisible 
     (curIdx, shd, stl, lhd, startLine, curLine, maxW, maxH, newCursorIdx) =
+      if startLine = curLine then
+        helpIsCursorVisible
+          (0, shd, stl, curIdx, maxW, maxH, 0, 0, newCursorIdx)
+      else
         let
           val relativeLine = (curLine + Vector.length lhd) - startLine
-        val lineIdx = Cursor.binSearch (relativeLine, lhd)
+          val lineIdx = Vector.sub (lhd, relativeLine)
           val absIdx = curIdx + lineIdx
         in
           helpIsCursorVisible