fix(cortex-tui): ensure settings list scrolls correctly in small terminals

Fixes bounty issue #1709

The interactive list renderer now computes the correct scroll offset at
render time based on the actual viewport height. Previously, scrolling
used only the max_visible value which could be larger than the terminal
window, causing items to not be visible when selected.

Author: factorydroid

cortex-tui/src/interactive/renderer.rs

@@ -123,8 +123,23 @@ impl<'a> InteractiveWidget<'a> {
     /// Render the list items.
     fn render_items(&self, area: Rect, buf: &mut Buffer) {
         let visible_items = self.state.visible_items();
-        let start = self.state.scroll_offset;
-        let end = (start + area.height as usize).min(visible_items.len());
+        let viewport_height = area.height as usize;
+
+        // Compute the effective scroll offset to ensure the selected item is visible.
+        // This handles cases where the actual viewport is smaller than max_visible,
+        // which can occur in small terminal windows.
+        let start = if self.state.selected >= self.state.scroll_offset + viewport_height {
+            // Selected item is below the visible area - scroll down
+            self.state.selected.saturating_sub(viewport_height - 1)
+        } else if self.state.selected < self.state.scroll_offset {
+            // Selected item is above the visible area - scroll up
+            self.state.selected
+        } else {
+            // Selected item is within view - use existing scroll offset
+            self.state.scroll_offset
+        };
+
+        let end = (start + viewport_height).min(visible_items.len());
 
         for (i, (real_idx, item)) in visible_items
             .iter()
@@ -329,4 +344,41 @@ mod tests {
         // 2 items + 1 title + 1 search + 1 hints + 2 border = 7
         assert_eq!(widget.required_height(), 7);
     }
+
+    #[test]
+    fn test_scroll_offset_calculation_small_viewport() {
+        // Test that scroll offset is computed correctly when viewport is smaller than max_visible.
+        // This tests the fix for issue #1709 where items couldn't be scrolled to in small terminals.
+        let items: Vec<InteractiveItem> = (0..20)
+            .map(|i| InteractiveItem::new(format!("{}", i), format!("Item {}", i)))
+            .collect();
+
+        let mut state =
+            InteractiveState::new("Test", items, InteractiveAction::Custom("test".into()))
+                .with_max_visible(25); // max_visible is 25, but viewport will be smaller
+
+        // Select the last item (index 19)
+        for _ in 0..19 {
+            state.select_next();
+        }
+        assert_eq!(state.selected, 19);
+
+        // Simulate a small viewport of height 5
+        let viewport_height: usize = 5;
+
+        // Calculate start as the renderer would
+        let start = if state.selected >= state.scroll_offset + viewport_height {
+            state.selected.saturating_sub(viewport_height - 1)
+        } else if state.selected < state.scroll_offset {
+            state.selected
+        } else {
+            state.scroll_offset
+        };
+
+        // With selected=19 and viewport_height=5, start should be 15
+        // so items 15-19 are visible, including the selected item 19
+        assert_eq!(start, 15);
+        assert!(state.selected >= start);
+        assert!(state.selected < start + viewport_height);
+    }
 }