scrolling window centered

src/main.rs

@@ -217,7 +217,7 @@ fn handle_key(app: &mut App, code: KeyCode) {
             _ => return,
         }
     }
-    
+
     match code {
         KeyCode::Char('q') => app.should_exit = true,
 
@@ -298,9 +298,34 @@ fn ui(frame: &mut Frame, app: &App) {
     );
 }
 
+fn compute_window(len: usize, selected: usize, viewport: usize) -> (usize, usize) {
+    if len == 0 || viewport == 0 {
+        return (0, 0);
+    }
+
+    let viewport = viewport.min(len);
+    let half = viewport / 2;
+
+    let mut start = selected.saturating_sub(half);
+    if start + viewport > len {
+        start = len - viewport;
+    }
+
+    let end = start + viewport;
+    (start, end)
+}
+
+
 fn render_categories(frame: &mut Frame, app: &App, left: Rect, right: Rect) {
-    let items: Vec<ListItem> = app
+    // how many rows can we show inside the bordered block?
-        .items
+    let viewport = left.height.saturating_sub(2) as usize;
+
+    let len = app.items.len();
+    let selected = app.selected.min(len.saturating_sub(1));
+
+    let (start, end) = compute_window(len, selected, viewport);
+
+    let items: Vec<ListItem> = app.items[start..end]
         .iter()
         .map(|s| ListItem::new(s.as_str()))
         .collect();
@@ -311,11 +336,17 @@ fn render_categories(frame: &mut Frame, app: &App, left: Rect, right: Rect) {
         .highlight_style(Style::default().add_modifier(Modifier::REVERSED));
 
     let mut state = ListState::default();
-    state.select(Some(app.selected));
+    state.select(Some(selected - start)); // selection relative to slice
 
     frame.render_stateful_widget(list, left, &mut state);
 
-    let detail = format!("Details for:\n\n{}", app.items[app.selected]);
+    // Right pane details (use the real selected item)
+    let detail = if len > 0 {
+        format!("Details for:\n\n{}", app.items[selected])
+    } else {
+        "No items".to_string()
+    };
+
     frame.render_widget(
         Paragraph::new(detail)
             .block(Block::bordered().title("Details").merge_borders(MergeStrategy::Exact)),