3860 - stop buffering the screen in termbox

To achieve this we have to switch to a model of the screen in termbox that
is closer to the underlying terminal.

Before:
  a screen is a grid of characters
  writing out of bounds does nothing

After:
  a screen is a scrolling raster of characters
  writing out of bounds wraps to next line and scrolls if necessary

To move to the new model, it was essential that I migrate my fake screen
at the same time to mimic it. This is why the first attempt (commit 3824)
failed (commit 3858). This is also why this commit can't be split into
smaller pieces.

The fake screen now 'scrolls' by rotating screen lines from top to bottom.
There's still no notion of a scrollback buffer.

The newer model is richer; it permits repl-like apps that upstream termbox
can't do easily. It also permits us to simply use `printf` or `cout` to
write to the screen, and everything mostly works as you would expect. Exceptions:

  a) '\n' won't do what you expect. You need to explicitly print both '\n'
  and '\r'.

  b) backspace won't do what you expect. It only moves the cursor back,
  without erasing the previous character. It does not wrap.

  Both behaviors exactly mimic my existing terminal's emulation of vt100.

The catch: it's easy to accidentally scroll in apps. Out-of-bounds prints
didn't matter before, but they're bugs now. To help track them down, use
the `save-top-idx`, `assert-no-scroll` pair of helpers.

  An important trick is to wrap the cursor before rather after printing
  a character. Otherwise we end up scrolling every time we print to the
  bottom-right character. This means that the cursor position can be invalid
  at the start of a print, and we need to handle that.

In the process we also lose the ability to hide and show the screen. We
have to show the prints happening. Seems apt for a "white-box" platform
like Mu.

1 files changed, 3 insertions, 5 deletions

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