fix(tui): clear first inline viewport render

codex-rs/tui/src/tui.rs

@@ -3,6 +3,7 @@ use std::future::Future;
 use std::io::IsTerminal;
 use std::io::Result;
 use std::io::Stdout;
+use std::io::Write;
 use std::io::stdin;
 use std::io::stdout;
 use std::panic;
@@ -76,8 +77,15 @@ fn should_emit_notification(condition: NotificationCondition, terminal_focused:
 
 #[cfg(test)]
 mod tests {
+    use std::io::Write as _;
+
+    use super::clear_for_viewport_change;
     use super::should_emit_notification;
+    use crate::custom_terminal::Terminal as CustomTerminal;
+    use crate::test_backend::VT100Backend;
     use codex_config::types::NotificationCondition;
+    use ratatui::layout::Position;
+    use ratatui::layout::Rect;
 
     #[test]
     fn unfocused_notification_condition_is_suppressed_when_focused() {
@@ -102,6 +110,47 @@ mod tests {
             /*terminal_focused*/ false
         ));
     }
+
+    #[test]
+    fn first_viewport_change_clears_from_new_viewport_when_old_viewport_is_empty() {
+        let width = 12;
+        let height = 4;
+        let backend = VT100Backend::new(width, height);
+        let mut terminal =
+            CustomTerminal::with_options_and_cursor_position(backend, Position { x: 0, y: 1 })
+                .expect("terminal");
+        write!(
+            terminal.backend_mut(),
+            "shell line\r\nstale cells\r\nmore stale"
+        )
+        .expect("prefill terminal");
+
+        clear_for_viewport_change(
+            &mut terminal,
+            Rect::new(
+                /*x*/ 0,
+                /*y*/ 1,
+                /*width*/ width,
+                /*height*/ height - 1,
+            ),
+        )
+        .expect("clear transition");
+
+        let rows: Vec<String> = terminal
+            .backend()
+            .vt100()
+            .screen()
+            .rows(/*start*/ 0, width)
+            .collect();
+        assert!(
+            rows[0].contains("shell line"),
+            "expected content before the viewport to remain visible, rows: {rows:?}"
+        );
+        assert!(
+            !rows.iter().skip(1).any(|row| row.contains("stale")),
+            "expected stale cells inside the new viewport to be cleared, rows: {rows:?}"
+        );
+    }
 }
 
 pub fn set_modes() -> Result<()> {
@@ -391,6 +440,18 @@ struct PendingHistoryLines {
     wrap_policy: HistoryLineWrapPolicy,
 }
 
+fn clear_for_viewport_change<B>(terminal: &mut CustomTerminal<B>, new_area: Rect) -> Result<()>
+where
+    B: Backend + Write,
+{
+    let clear_position = if terminal.viewport_area.is_empty() {
+        new_area.as_position()
+    } else {
+        terminal.viewport_area.as_position()
+    };
+    terminal.clear_after_position(clear_position)
+}
+
 impl Tui {
     pub fn new(terminal: Terminal) -> Self {
         let (draw_tx, _) = broadcast::channel(1);
@@ -642,8 +703,9 @@ impl Tui {
             area.y = size.height - area.height;
         }
         if area != terminal.viewport_area {
-            // TODO(nornagon): probably this could be collapsed with the clear + set_viewport_area above.
-            terminal.clear()?;
+            // On startup, the old viewport can still be empty. Clear from the
+            // new viewport top so stale shell cells do not show through spaces.
+            clear_for_viewport_change(terminal, area)?;
             terminal.set_viewport_area(area);
         }