streamline ui (#1733)

Simplify and improve many UI elements.
* Remove all-around borders in most places. These interact badly with
terminal resizing and look heavy. Prefer left-side-only borders.
* Make the viewport adjust to the size of its contents.
* <kbd>/</kbd> and <kbd>@</kbd> autocomplete boxes appear below the
prompt, instead of above it.
* Restyle the keyboard shortcut hints & move them to the left.
* Restyle the approval dialog.
* Use synchronized rendering to avoid flashing during rerenders.


https://github.com/user-attachments/assets/96f044af-283b-411c-b7fc-5e6b8a433c20

<img width="1117" height="858" alt="Screenshot 2025-07-30 at 5 29 20 PM"
src="https://github.com/user-attachments/assets/0cc0af77-8396-429b-b6ee-9feaaccdbee7"
/>

codex-rs/tui/src/bottom_pane/file_search_popup.rs

@@ -115,12 +115,8 @@ impl FileSearchPopup {
         // row so the popup is still visible. When matches are present we show
         // up to MAX_RESULTS regardless of the waiting flag so the list
         // remains stable while a newer search is in-flight.
-        let rows = if self.matches.is_empty() {
-            1
-        } else {
-            self.matches.len().clamp(1, MAX_RESULTS)
-        } as u16;
-        rows + 2 // border
+
+        self.matches.len().clamp(1, MAX_RESULTS) as u16
     }
 }
 
@@ -128,7 +124,14 @@ impl WidgetRef for &FileSearchPopup {
     fn render_ref(&self, area: Rect, buf: &mut Buffer) {
         // Prepare rows.
         let rows: Vec<Row> = if self.matches.is_empty() {
-            vec![Row::new(vec![Cell::from(" no matches ")])]
+            vec![Row::new(vec![
+                Cell::from(if self.waiting {
+                    "(searching …)"
+                } else {
+                    "no matches"
+                })
+                .style(Style::new().add_modifier(Modifier::ITALIC | Modifier::DIM)),
+            ])]
         } else {
             self.matches
                 .iter()
@@ -169,17 +172,12 @@ impl WidgetRef for &FileSearchPopup {
                 .collect()
         };
 
-        let mut title = format!(" @{} ", self.pending_query);
-        if self.waiting {
-            title.push_str(" (searching …)");
-        }
-
         let table = Table::new(rows, vec![Constraint::Percentage(100)])
             .block(
                 Block::default()
-                    .borders(Borders::ALL)
-                    .border_type(BorderType::Rounded)
-                    .title(title),
+                    .borders(Borders::LEFT)
+                    .border_type(BorderType::QuadrantOutside)
+                    .border_style(Style::default().fg(Color::DarkGray)),
             )
             .widths([Constraint::Percentage(100)]);