resizable viewport (#1732)

Proof of concept for a resizable viewport.

The general approach here is to duplicate the `Terminal` struct from
ratatui, but with our own logic. This is a "light fork" in that we are
still using all the base ratatui functions (`Buffer`, `Widget` and so
on), but we're doing our own bookkeeping at the top level to determine
where to draw everything.

This approach could use improvement—e.g, when the window is resized to a
smaller size, if the UI wraps, we don't correctly clear out the
artifacts from wrapping. This is possible with a little work (i.e.
tracking what parts of our UI would have been wrapped), but this
behavior is at least at par with the existing behavior.


https://github.com/user-attachments/assets/4eb17689-09fd-4daa-8315-c7ebc654986d


cc @joshka who might have Thoughts™

codex-rs/tui/src/tui.rs

@@ -5,14 +5,13 @@ use std::io::stdout;
 use codex_core::config::Config;
 use crossterm::event::DisableBracketedPaste;
 use crossterm::event::EnableBracketedPaste;
-use ratatui::Terminal;
-use ratatui::TerminalOptions;
-use ratatui::Viewport;
 use ratatui::backend::CrosstermBackend;
 use ratatui::crossterm::execute;
 use ratatui::crossterm::terminal::disable_raw_mode;
 use ratatui::crossterm::terminal::enable_raw_mode;
 
+use crate::custom_terminal::Terminal;
+
 /// A type alias for the terminal type used in this application
 pub type Tui = Terminal<CrosstermBackend<Stdout>>;
 
@@ -23,19 +22,8 @@ pub fn init(_config: &Config) -> Result<Tui> {
     enable_raw_mode()?;
     set_panic_hook();
 
-    // Reserve a fixed number of lines for the interactive viewport (composer,
-    // status, popups). History is injected above using `insert_before`. This
-    // is an initial step of the refactor – later the height can become
-    // dynamic. For now a conservative default keeps enough room for the
-    // multi‑line composer while not occupying the whole screen.
-    const BOTTOM_VIEWPORT_HEIGHT: u16 = 8;
     let backend = CrosstermBackend::new(stdout());
-    let tui = Terminal::with_options(
-        backend,
-        TerminalOptions {
-            viewport: Viewport::Inline(BOTTOM_VIEWPORT_HEIGHT),
-        },
-    )?;
+    let tui = Terminal::with_options(backend)?;
     Ok(tui)
 }