implement smoother horizontal scrolling. If the new column is already visible without changing the previous scroll column, then don't change. If the new scroll column is prior to the old, then scroll backwards; else, scroll forwards.

fcore/text-scroll.sml

@@ -3,23 +3,26 @@ struct
   structure TC = TextConstants
 
   (* Preqreuisite: move buffer to cursorIdx *)
-  fun getScrollColumn (buffer, cursorIdx, windowWidth) =
+  fun getScrollColumn (buffer, cursorIdx, windowWidth, prevScrollColumn) =
     let
       val startOfLine = Cursor.vi0 (buffer, cursorIdx)
-      val columnDifference = cursorIdx - startOfLine
+      val newColumn = cursorIdx - startOfLine
+      val howManyColumnsCanWeFit =
+        if windowWidth >= TC.textLineWidth then TC.textLineCount
+        else windowWidth div TC.xSpace
+      val howManyColumnsCanWeFit = howManyColumnsCanWeFit - 1
     in
-      if columnDifference = 0 then
-        0
+      if newColumn < prevScrollColumn then
+        (* we are moving the cursor backwards
+         * so make sure that newColumn is on the left side *)
+        newColumn
+      else if newColumn > prevScrollColumn + howManyColumnsCanWeFit then
+        (* we are scrolling forwards *)
+        newColumn - howManyColumnsCanWeFit
       else
-        let
-          val howManyColumnsCanWeFit =
-            if windowWidth >= TC.textLineWidth then TC.textLineCount
-            else windowWidth div TC.xSpace
-          val howManyColumnsCanWeFit = howManyColumnsCanWeFit - 1
-        in
-          if columnDifference < howManyColumnsCanWeFit then 0
-          else columnDifference - howManyColumnsCanWeFit
-        end
+        (* we can display the current column without moving the scroll column
+         * so we do that *)
+        prevScrollColumn
     end
 
   fun getStartLine (prevLineNumber, cursorLine, windowHeight) =