fix bugs moving image up/down in move mode

fcore/move-mode.sml

@@ -5,7 +5,7 @@ struct
   open DrawMessage
   open UpdateMessage
 
-  (* todo: resize message, escape button to go back to normal mode *)
+  (* todo: resize message *)
 
   fun getDrawMsg (model: app_type) =
     let
@@ -37,8 +37,13 @@ struct
       (model, [DRAW drawMsg])
     end
 
+  val blankPixel = {r = 0, g = 0, b = 0, a = 0}
+
   fun makeBlankYAxis length =
-    Vector.tabulate (length, fn _ => {r = 0, g = 0, b = 0, a = 0})
+    Vector.tabulate (length, fn _ => blankPixel)
+
+  fun makeBlankXAxis length =
+    Vector.tabulate (length, fn _ => makeBlankYAxis length)
 
   fun moveImageUp (model: app_type) =
     let
@@ -46,11 +51,11 @@ struct
 
       val squares =
         Vector.mapi
-          (fn (idx, yAxis) =>
+          (fn (_, yAxis) =>
-             if idx + 1 < Vector.length squares then
+             Vector.mapi
-               Vector.sub (squares, idx + 1)
+               (fn (yIdx, pixel) =>
-             else
+                  if yIdx = Vector.length yAxis - 1 then blankPixel
-               makeBlankYAxis (Vector.length yAxis)) squares
+                  else Vector.sub (yAxis, yIdx + 1)) yAxis) squares
 
       val model = AppWith.squares (model, squares)
     in
@@ -63,9 +68,11 @@ struct
 
       val squares =
         Vector.mapi
-          (fn (idx, yAxis) =>
+          (fn (_, yAxis) =>
-             if idx = 0 then makeBlankYAxis (Vector.length yAxis)
+             Vector.mapi
-             else Vector.sub (squares, idx - 1)) squares
+               (fn (yIdx, pixel) =>
+                  if yIdx = 0 then blankPixel else Vector.sub (yAxis, yIdx - 1))
+               yAxis) squares
 
       val model = AppWith.squares (model, squares)
     in