extract a reusable function to calculate the scroll column in different contexts (can be used for a plain string or a LineGap.t; doesn't matter)

fcore/text-scroll.sml

@@ -2,10 +2,10 @@ structure TextScroll =
 struct
   structure TC = TextConstants
 
-  (* Preqreuisite: move buffer to cursorIdx *)
-  fun getScrollColumn (buffer, cursorIdx, windowWidth, prevScrollColumn) =
+  (* calculates new scroll column from integer arguments *)
+  fun calculateScrollColumn
+    (startOfLine, cursorIdx, windowWidth, prevScrollColumn) =
     let
-      val startOfLine = Cursor.vi0 (buffer, cursorIdx)
       val newColumn = cursorIdx - startOfLine
       val howManyColumnsCanWeFit =
         if windowWidth >= TC.textLineWidth then TC.textLineCount
@@ -25,6 +25,15 @@ struct
         prevScrollColumn
     end
 
+  (* Preqreuisite: move buffer to cursorIdx *)
+  fun getScrollColumn (buffer, cursorIdx, windowWidth, prevScrollColumn) =
+    let
+      val startOfLine = Cursor.vi0 (buffer, cursorIdx)
+    in
+      calculateScrollColumn
+        (startOfLine, cursorIdx, windowWidth, prevScrollColumn)
+    end
+
   fun getStartLine (prevLineNumber, cursorLine, windowHeight) =
     if cursorLine <= (prevLineNumber + 3) then
       (* cursorLine is prior to or same as prevLineNumber,