add new function to calculate the startLine

fcore/normal-mode/normal-finish.sml

@@ -17,7 +17,6 @@ struct
       (* calculate scroll column *)
       val buffer = LineGap.goToIdx (cursorIdx, buffer)
       val visualScrollColumn = TextScroll.getScrollColumn (buffer, cursorIdx, windowWidth)
-      val () = print (Int.toString visualScrollColumn ^ "\n")
 
       (* move LineGap to first line displayed on screen *)
       val buffer = LineGap.goToLine (startLine, buffer)

fcore/text-scroll.sml

@@ -17,8 +17,25 @@ struct
             else windowWidth div TC.xSpace
         in
           if columnDifference < howManyColumnsCanWeFit then 0
-          else
+          else columnDifference - howManyColumnsCanWeFit
-            columnDifference - howManyColumnsCanWeFit
         end
     end
+
+  fun getStartLine (prevLineNumber, cursorLine, windowHeight) =
+    if cursorLine <= prevLineNumber then
+      (* if cursorLine is prior or same as prevLineNumber, 
+       * then use cursorLine as scroll-line-start *)
+      cursorLine
+    else
+      (* cursorLine > prevLineNumber *)
+      let
+        val howManyLinesWeCanFit = windowHeight div TC.ySpace
+      in
+        if prevLineNumber + howManyLinesWeCanFit >= cursorLine then
+          prevLineNumber
+        else
+          let val lineDifference = cursorLine - prevLineNumber
+          in prevLineNumber + lineDifference
+          end
+      end
 end