in TextBuilder.build functions, when we encounter a space, check if we are before the scroll column's start, and set the x position to startX if so. This is done in preparation for the character which is actually at the scroll column's start, which will be added at the wrong offset unless we do this.

fcore/text-builder/text-builder-with-cursor.sml

@@ -168,6 +168,14 @@ struct
                 Utils.makeCursor (posX, posY, env) :: acc
               else
                 acc
+            val posX =
+              if column < #scrollColumnStart env then
+                (* if we are prior to the start column, 
+                 * we want to set the x position to be at the start
+                 * in preparation for when we are at the start column *)
+                #startX env
+              else
+                posX + TC.xSpace
           in
             build
               ( pos + 1
@@ -175,7 +183,7 @@ struct
               , stl
               , line
               , ltl
-              , posX + TC.xSpace
+              , posX
               , posY
               , column + 1
               , lineNumber

fcore/text-builder/text-builder-with-highlight.sml

@@ -208,6 +208,9 @@ struct
                     Utils.makeHighlight (posX, posY, env) :: acc
                   else
                     acc
+                val posX =
+                  if column < #scrollColumnStart env then #startX env
+                  else posX + TC.xSpace
               in
                 build
                   ( pos + 1
@@ -215,7 +218,7 @@ struct
                   , stl
                   , line
                   , ltl
-                  , posX + TC.xSpace
+                  , posX
                   , posY
                   , column + 1
                   , lineNumber