fix getScrollColumn calculation: the right calculation is to subtract the column number by how many columns we can fit

fcore/normal-mode/normal-finish.sml

@@ -17,6 +17,7 @@ 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

@@ -16,7 +16,9 @@ struct
             if windowWidth >= TC.textLineWidth then TC.textLineCount
             else windowWidth div TC.xSpace
         in
-          columnDifference mod howManyColumnsCanWeFit
+          if columnDifference < howManyColumnsCanWeFit then 0
+          else
+            columnDifference - howManyColumnsCanWeFit
         end
     end
 end