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/insert_history.rs

@@ -4,6 +4,7 @@ use std::io::Write;
 
 use crate::tui;
 use crossterm::Command;
+use crossterm::cursor::MoveTo;
 use crossterm::queue;
 use crossterm::style::Color as CColor;
 use crossterm::style::Colors;
@@ -12,7 +13,6 @@ use crossterm::style::SetAttribute;
 use crossterm::style::SetBackgroundColor;
 use crossterm::style::SetColors;
 use crossterm::style::SetForegroundColor;
-use ratatui::layout::Position;
 use ratatui::layout::Size;
 use ratatui::prelude::Backend;
 use ratatui::style::Color;
@@ -23,6 +23,7 @@ use ratatui::text::Span;
 /// Insert `lines` above the viewport.
 pub(crate) fn insert_history_lines(terminal: &mut tui::Tui, lines: Vec<Line>) {
     let screen_size = terminal.backend().size().unwrap_or(Size::new(0, 0));
+    let cursor_pos = terminal.get_cursor_position().ok();
 
     let mut area = terminal.get_frame().area();
 
@@ -60,9 +61,10 @@ pub(crate) fn insert_history_lines(terminal: &mut tui::Tui, lines: Vec<Line>) {
     // └──────────────────────────────┘
     queue!(std::io::stdout(), SetScrollRegion(1..area.top())).ok();
 
-    terminal
-        .set_cursor_position(Position::new(0, cursor_top))
-        .ok();
+    // NB: we are using MoveTo instead of set_cursor_position here to avoid messing with the
+    // terminal's last_known_cursor_position, which hopefully will still be accurate after we
+    // fetch/restore the cursor position. insert_history_lines should be cursor-position-neutral :)
+    queue!(std::io::stdout(), MoveTo(0, cursor_top)).ok();
 
     for line in lines {
         queue!(std::io::stdout(), Print("\r\n")).ok();
@@ -70,6 +72,11 @@ pub(crate) fn insert_history_lines(terminal: &mut tui::Tui, lines: Vec<Line>) {
     }
 
     queue!(std::io::stdout(), ResetScrollRegion).ok();
+
+    // Restore the cursor position to where it was before we started.
+    if let Some(cursor_pos) = cursor_pos {
+        queue!(std::io::stdout(), MoveTo(cursor_pos.x, cursor_pos.y)).ok();
+    }
 }
 
 fn wrapped_line_count(lines: &[Line], width: u16) -> u16 {