9 files changed, 8 insertions, 21 deletions
diff --git a/sandbox/001-editor.mu b/sandbox/001-editor.mu
index d81278ae..a6dde85b 100644
--- a/sandbox/001-editor.mu
+++ b/sandbox/001-editor.mu
@@ -6,10 +6,8 @@ def main text:text [
   local-scope
   load-ingredients
   open-console
-  hide-screen 0/screen
   e:&:editor <- new-editor text, 0/left, 5/right
   render 0/screen, e
-  show-screen 0/screen
   wait-for-event 0/console
   close-console
 ]
diff --git a/sandbox/002-typing.mu b/sandbox/002-typing.mu
index 8c48f147..16693429 100644
--- a/sandbox/002-typing.mu
+++ b/sandbox/002-typing.mu
@@ -264,6 +264,7 @@ def insert-at-cursor editor:&:editor, c:char, screen:&:screen -> go-render?:bool
 def editor-render screen:&:screen, editor:&:editor -> screen:&:screen, editor:&:editor [
   local-scope
   load-ingredients
+  old-top-idx:num <- save-top-idx screen
   left:num <- get *editor, left:offset
   right:num <- get *editor, right:offset
   row:num, column:num <- render screen, editor
@@ -272,6 +273,7 @@ def editor-render screen:&:screen, editor:&:editor -> screen:&:screen, editor:&:
   draw-horizontal screen, row, left, right, 9480/horizontal-dotted
   row <- add row, 1
   clear-screen-from screen, row, left, left, right
+  assert-no-scroll screen, old-top-idx
 ]
 
 scenario editor-handles-empty-event-queue [
@@ -1057,6 +1059,9 @@ after <handle-special-key> [
 def draw-horizontal screen:&:screen, row:num, x:num, right:num -> screen:&:screen [
   local-scope
   load-ingredients
+  height:num <- screen-height screen
+  past-bottom?:bool <- greater-or-equal row, height
+  return-if past-bottom?
   style:char, style-found?:bool <- next-ingredient
   {
     break-if style-found?
diff --git a/sandbox/004-programming-environment.mu b/sandbox/004-programming-environment.mu
index 0f1280e3..53f9f846 100644
--- a/sandbox/004-programming-environment.mu
+++ b/sandbox/004-programming-environment.mu
@@ -79,7 +79,6 @@ def event-loop screen:&:screen, console:&:console, env:&:environment, resources:
     }
     # not global and not a touch event
     {
-      hide-screen screen
       render?:bool <- handle-keyboard-event screen, current-sandbox, e:event
       break-unless render?
       # try to batch up rendering if there are more events queued up
@@ -93,7 +92,6 @@ def event-loop screen:&:screen, console:&:console, env:&:environment, resources:
       }
       +finish-event
       screen <- update-cursor screen, current-sandbox, env
-      show-screen screen
     }
     loop
   }
@@ -199,7 +197,7 @@ def render-all screen:&:screen, env:&:environment, {render-editor: (recipe (addr
   local-scope
   load-ingredients
   trace 10, [app], [render all]
-  hide-screen screen
+  old-top-idx:num <- save-top-idx screen
   # top menu
   trace 11, [app], [render top menu]
   width:num <- screen-width screen
@@ -211,12 +209,12 @@ def render-all screen:&:screen, env:&:environment, {render-editor: (recipe (addr
   print screen, [ run (F4) ], 255/white, 161/reddish
   #
   screen <- render-sandbox-side screen, env, render-editor
-  <render-components-end>
+  <render-components-end>  # no early returns permitted
   #
   current-sandbox:&:editor <- get *env, current-sandbox:offset
   screen <- update-cursor screen, current-sandbox, env
   #
-  show-screen screen
+  assert-no-scroll screen, old-top-idx
 ]
 
 # replaced in a later layer
diff --git a/sandbox/005-sandbox.mu b/sandbox/005-sandbox.mu
index e9500797..a7df7a54 100644
--- a/sandbox/005-sandbox.mu
+++ b/sandbox/005-sandbox.mu
@@ -793,9 +793,7 @@ after <global-keypress> [
       render-from <- add render-from, 1
       *env <- put *env, render-from:offset, render-from
     }
-    hide-screen screen
     screen <- render-sandbox-side screen, env, render
-    show-screen screen
     jump +finish-event
   }
 ]
@@ -822,9 +820,7 @@ after <global-keypress> [
     break-if at-beginning?
     render-from <- subtract render-from, 1
     *env <- put *env, render-from:offset, render-from
-    hide-screen screen
     screen <- render-sandbox-side screen, env, render
-    show-screen screen
     jump +finish-event
   }
 ]
diff --git a/sandbox/006-sandbox-copy.mu b/sandbox/006-sandbox-copy.mu
index 995f4c7c..4835f02e 100644
--- a/sandbox/006-sandbox-copy.mu
+++ b/sandbox/006-sandbox-copy.mu
@@ -140,10 +140,8 @@ after <global-touch> [
     break-unless copy?
     copy?, env <- try-copy-sandbox click-row, env
     break-unless copy?
-    hide-screen screen
     screen <- render-sandbox-side screen, env, render
     screen <- update-cursor screen, current-sandbox, env
-    show-screen screen
     loop +next-event
   }
 ]
diff --git a/sandbox/007-sandbox-delete.mu b/sandbox/007-sandbox-delete.mu
index ddfbf692..107c861c 100644
--- a/sandbox/007-sandbox-delete.mu
+++ b/sandbox/007-sandbox-delete.mu
@@ -69,10 +69,8 @@ after <global-touch> [
     break-unless delete?
     delete?, env <- try-delete-sandbox click-row, env
     break-unless delete?
-    hide-screen screen
     screen <- render-sandbox-side screen, env, render
     screen <- update-cursor screen, current-sandbox, env
-    show-screen screen
     loop +next-event
   }
 ]
diff --git a/sandbox/008-sandbox-edit.mu b/sandbox/008-sandbox-edit.mu
index cb19ebc4..ec4fd578 100644
--- a/sandbox/008-sandbox-edit.mu
+++ b/sandbox/008-sandbox-edit.mu
@@ -111,10 +111,8 @@ after <global-touch> [
     break-unless edit?
     edit?, env <- try-edit-sandbox click-row, env
     break-unless edit?
-    hide-screen screen
     screen <- render-sandbox-side screen, env, render
     screen <- update-cursor screen, current-sandbox, env
-    show-screen screen
     loop +next-event
   }
 ]
diff --git a/sandbox/009-sandbox-test.mu b/sandbox/009-sandbox-test.mu
index d7b8ed62..1c24bcb8 100644
--- a/sandbox/009-sandbox-test.mu
+++ b/sandbox/009-sandbox-test.mu
@@ -132,10 +132,8 @@ after <global-touch> [
     # toggle its expected-response, and save session
     sandbox <- toggle-expected-response sandbox
     save-sandboxes env, resources
-    hide-screen screen
     screen <- render-sandbox-side screen, env, render
     screen <- update-cursor screen, current-sandbox, env
-    show-screen screen
     loop +next-event
   }
 ]
diff --git a/sandbox/010-sandbox-trace.mu b/sandbox/010-sandbox-trace.mu
index f81d4151..27f2915a 100644
--- a/sandbox/010-sandbox-trace.mu
+++ b/sandbox/010-sandbox-trace.mu
@@ -190,10 +190,8 @@ after <global-touch> [
     x:bool <- get *sandbox, display-trace?:offset
     x <- not x
     *sandbox <- put *sandbox, display-trace?:offset, x
-    hide-screen screen
     screen <- render-sandbox-side screen, env, render
     screen <- update-cursor screen, current-sandbox, env
-    show-screen screen
     loop +next-event
   }
 ]