feat: Ctrl+J for newline in Rust TUI, default to one line of height (#926)

While the `TextArea` used in the Rust TUI is "multiline," it is not like
an HTML `<textarea>` in that it does not wrap, so there was not much
benefit to setting `MIN_TEXTAREA_ROWS` to `3`, so this PR changes it to
`1`. Though there are now three ways to "increase" the height due to
actual linebreaks:

* paste in multiline content (this worked before this PR)
* pressing `Ctrl+J` will insert a newline
* if you have your terminal emulator set such that it is possible to
press something that `crossterm` interprets as "Enter plus some
modifier," then now that will also work

Now things look a bit more compact on startup:

<img width="745" alt="image"
src="https://github.com/user-attachments/assets/86e2857f-f31c-46f5-a80b-1ab2120b266e"
/>

codex-rs/tui/src/bottom_pane.rs

@@ -29,7 +29,7 @@ use crate::user_approval_widget::ApprovalRequest;
 use crate::user_approval_widget::UserApprovalWidget;
 
 /// Minimum number of visible text rows inside the textarea.
-const MIN_TEXTAREA_ROWS: usize = 3;
+const MIN_TEXTAREA_ROWS: usize = 1;
 /// Number of terminal rows consumed by the textarea border (top + bottom).
 const TEXTAREA_BORDER_LINES: u16 = 2;
 
@@ -176,6 +176,24 @@ impl<'a> BottomPane<'a> {
                         self.request_redraw()?;
                         Ok(InputResult::Submitted(text))
                     }
+                    Input {
+                        key: Key::Enter, ..
+                    }
+                    | Input {
+                        key: Key::Char('j'),
+                        ctrl: true,
+                        alt: false,
+                        shift: false,
+                    } => {
+                        // If the user has their terminal emulator configured so
+                        // Enter+Shift (or any modifier) sends a different key
+                        // event, we should let them insert a newline.
+                        //
+                        // We also allow Ctrl+J to insert a newline.
+                        self.textarea.insert_newline();
+                        self.request_redraw()?;
+                        Ok(InputResult::None)
+                    }
                     input => {
                         self.textarea.input(input);
                         self.request_redraw()?;
@@ -284,7 +302,6 @@ impl WidgetRef for &BottomPane<'_> {
 // for all variants of PaneState.
 fn update_border_for_input_focus(textarea: &mut TextArea, state: &PaneState, has_focus: bool) {
     struct BlockState {
-        title: &'static str,
         right_title: Line<'static>,
         border_style: Style,
     }
@@ -297,26 +314,23 @@ fn update_border_for_input_focus(textarea: &mut TextArea, state: &PaneState, has
 
     let block_state = if has_focus && accepting_input {
         BlockState {
-            title: "use Enter to send for now (Ctrl-D to quit)",
+            right_title: Line::from("Enter to send | Ctrl+D to quit | Ctrl+J for newline")
-            right_title: Line::from("press enter to send").alignment(Alignment::Right),
+                .alignment(Alignment::Right),
             border_style: Style::default(),
         }
     } else {
         BlockState {
-            title: "",
             right_title: Line::from(""),
             border_style: Style::default().dim(),
         }
     };
 
     let BlockState {
-        title,
         right_title,
         border_style,
     } = block_state;
     textarea.set_block(
         ratatui::widgets::Block::default()
-            .title_bottom(title)
             .title_bottom(right_title)
             .borders(ratatui::widgets::Borders::ALL)
             .border_type(BorderType::Rounded)