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/normal-mode/normal-finish.sml

@@ -12,12 +12,13 @@ struct
   fun buildTextAndClear
     (app: app_type, buffer, cursorIdx, searchList, msgs, bufferModifyTime) =
     let
-      val {windowWidth, windowHeight, startLine, searchString, ...} = app
+      val {windowWidth, windowHeight, startLine, searchString,
+      visualScrollColumn = prevScrollColumn, ...} = app
 
       (* calculate scroll column *)
       val buffer = LineGap.goToIdx (cursorIdx, buffer)
       val visualScrollColumn =
-        TextScroll.getScrollColumn (buffer, cursorIdx, windowWidth)
+        TextScroll.getScrollColumn (buffer, cursorIdx, windowWidth, prevScrollColumn)
 
       (* move LineGap to first line displayed on screen *)
       val buffer = LineGap.goToLine (startLine, buffer)
@@ -86,12 +87,13 @@ struct
         , searchList
         , searchString
         , bufferModifyTime
+        , visualScrollColumn = prevScrollColumn
         , ...
         } = app
 
       val newBuffer = LineGap.goToIdx (cursorIdx, buffer)
       val visualScrollColumn =
-        TextScroll.getScrollColumn (buffer, cursorIdx, newWidth)
+        TextScroll.getScrollColumn (buffer, cursorIdx, newWidth, prevScrollColumn)
       val newBuffer = LineGap.goToLine (startLine, newBuffer)
       val lineIdx = TextBuilderUtils.getLineAbsIdxFromBuffer (startLine, buffer)
 
@@ -130,11 +132,12 @@ struct
     (app: app_type, buffer, cursorIdx, searchList, initialMsg, bufferModifyTime) :
     AppType.app_type =
     let
-      val {windowWidth, windowHeight, startLine, searchString, ...} = app
+      val {windowWidth, windowHeight, startLine, searchString, visualScrollColumn
+      = prevScrollColumn, ...} = app
 
       val buffer = LineGap.goToIdx (cursorIdx, buffer)
       val visualScrollColumn =
-        TextScroll.getScrollColumn (buffer, cursorIdx, windowWidth)
+        TextScroll.getScrollColumn (buffer, cursorIdx, windowWidth, prevScrollColumn)
 
       (* move LineGap to first line displayed on screen *)
       val buffer = LineGap.goToLine (startLine, buffer)

fcore/normal-mode/normal-move.sml

@@ -57,6 +57,7 @@ struct
         , searchList
         , searchString
         , bufferModifyTime
+        , visualScrollColumn = prevScrollColumn
         , ...
         } = app
 
@@ -69,7 +70,7 @@ struct
 
       val buffer = LineGap.goToIdx (bufferIdx, buffer)
       val visualScrollColumn =
-        TextScroll.getScrollColumn (buffer, bufferIdx, windowWidth)
+        TextScroll.getScrollColumn (buffer, bufferIdx, windowWidth, prevScrollColumn)
       val bufferLine =
         let
           val maxHeight = windowHeight - TextConstants.ySpace
@@ -124,6 +125,7 @@ struct
           , searchList
           , searchString
           , bufferModifyTime
+          , visualScrollColumn = prevScrollColumn
           , ...
           } = app
         val buffer = LineGap.goToLine (reqLine, buffer)
@@ -133,7 +135,7 @@ struct
 
         val buffer = LineGap.goToIdx (cursorIdx, buffer)
         val visualScrollColumn =
-          TextScroll.getScrollColumn (buffer, cursorIdx, windowWidth)
+          TextScroll.getScrollColumn (buffer, cursorIdx, windowWidth, prevScrollColumn)
         val startLine = TextWindow.getStartLineWithCursorCentered
           (buffer, cursorIdx, origLine, windowWidth, windowHeight div 2)
 
@@ -179,7 +181,7 @@ struct
         , searchList
         , searchString
         , bufferModifyTime
-        , visualScrollColumn
+        , visualScrollColumn = prevScrollColumn
         , ...
         } = app
 
@@ -193,7 +195,7 @@ struct
       let
         val buffer = LineGap.goToIdx (cursorIdx, buffer)
         val visualScrollColumn =
-          TextScroll.getScrollColumn (buffer, cursorIdx, windowWidth)
+          TextScroll.getScrollColumn (buffer, cursorIdx, windowWidth, prevScrollColumn)
         val startLine = TextWindow.getStartLineWithCursorCentered
           (buffer, cursorIdx, startLine, windowWidth, windowHeight div 2)
 

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
-    in
-      if columnDifference = 0 then
-        0
-      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
+      if newColumn < prevScrollColumn then
-          else columnDifference - howManyColumnsCanWeFit
+        (* we are moving the cursor backwards
-        end
+         * so make sure that newColumn is on the left side *)
+        newColumn
+      else if newColumn > prevScrollColumn + howManyColumnsCanWeFit then
+        (* we are scrolling forwards *)
+        newColumn - howManyColumnsCanWeFit
+      else
+        (* we can display the current column without moving the scroll column
+         * so we do that *)
+        prevScrollColumn
     end
 
   fun getStartLine (prevLineNumber, cursorLine, windowHeight) =