3954

As a blanket rule, down-arrow now stops scrolling once the bottom margin
comes on screen.

Now that we have page-wise scrolling with ctrl-f/b and line-wise
scrolling with ctrl-s/x, we don't need to conflate scroll positioning
with the arrow keys. And as a result, early students no longer have to
struggle with accidentally scrolling part of the sandbox off the screen
when there's tons of empty space available.

`move-to-next-line` is still super messy and will need further
rethinking, but this commit simplifies the codebase as a whole by
eliminating a couple of historical accidents:

  a) We only introduced scrolling past the bottom of the screen to allow
  more sandboxes to come into view before we had scrolling for the
  sandbox side.

  b) We undid scrolling past the bottom in just the recipe side to allow
  errors to come into view.

Since these historical details are now irrelevant, we no longer need
separate logic for the recipe and sandbox sides, and we don't need to
keep track of the recipe-bottom separate from the bottom margin of
arbitrary editors.

1 files changed, 26 insertions, 21 deletions

diff --git a/sandbox/003-shortcuts.mu b/sandbox/003-shortcuts.mu
index 6fdd70b8..adee48a7 100644
--- a/sandbox/003-shortcuts.mu
+++ b/sandbox/003-shortcuts.mu
@@ -1348,38 +1348,43 @@ def move-to-next-line editor:&:editor, screen-height:num -> editor:&:editor [
   left:num <- get *editor, left:offset
   right:num <- get *editor, right:offset
   last-line:num <- subtract screen-height, 1
+  bottom:num <- get *editor, bottom:offset
+  at-bottom-of-screen?:bool <- greater-or-equal bottom, last-line
+  return-unless before-cursor
+  next:&:duplex-list:char <- next before-cursor
+  return-unless next
   already-at-bottom?:bool <- greater-or-equal cursor-row, last-line
-    # incorrect indent to help diff with edit/
     # if cursor not at bottom, move it
     return-if already-at-bottom?
-    # scan to start of next line, then to right column or until end of line
-    max:num <- subtract right, left
-    next-line:&:duplex-list:char <- before-start-of-next-line before-cursor, max
-    # already at end of buffer? do nothing
-    no-motion?:bool <- equal next-line, before-cursor
-    return-if no-motion?
-    cursor-row <- add cursor-row, 1
-    *editor <- put *editor, cursor-row:offset, cursor-row
-    before-cursor <- copy next-line
-    *editor <- put *editor, before-cursor:offset, before-cursor
     target-column:num <- copy cursor-column
+    # scan to start of next line
+    {
+      next:&:duplex-list:char <- next before-cursor
+      break-unless next
+      done?:bool <- greater-or-equal cursor-column, right
+      break-if done?
+      cursor-column <- add cursor-column, 1
+      before-cursor <- copy next
+      c:char <- get *next, value:offset
+      at-newline?:bool <- equal c, 10/newline
+      break-if at-newline?
+      loop
+    }
+    return-unless next
+    cursor-row <- add cursor-row, 1
     cursor-column <- copy left
-    *editor <- put *editor, cursor-column:offset, cursor-column
     {
+      next:&:duplex-list:char <- next before-cursor
+      break-unless next
       done?:bool <- greater-or-equal cursor-column, target-column
       break-if done?
-      curr:&:duplex-list:char <- next before-cursor
-      break-unless curr
-      currc:char <- get *curr, value:offset
-      at-newline?:bool <- equal currc, 10/newline
-      break-if at-newline?
-      #
-      before-cursor <- copy curr
-      *editor <- put *editor, before-cursor:offset, before-cursor
       cursor-column <- add cursor-column, 1
-      *editor <- put *editor, cursor-column:offset, cursor-column
+      before-cursor <- copy next
       loop
     }
+    *editor <- put *editor, before-cursor:offset, before-cursor
+    *editor <- put *editor, cursor-column:offset, cursor-column
+    *editor <- put *editor, cursor-row:offset, cursor-row
 ]
 
 scenario editor-adjusts-column-at-next-line [