fix(tui): prevent underflow in dropdown navigation and scroll calculations

Fixes #5182, #5180, #5176

- Add guard for max_visible=0 in dropdown select_next/select_prev
- Use saturating_sub in scroll.ensure_visible to prevent underflow
- Add bounds checking in scrollable_dropdown visible_items
- Guard against max_visible=0 in select_next/select_prev/calculate_scroll_offset

Author: echobt

src/cortex-tui-components/src/dropdown.rs

@@ -99,7 +99,7 @@ impl DropdownState {
 
     /// Select the next item.
     pub fn select_next(&mut self) {
-        if self.items.is_empty() {
+        if self.items.is_empty() || self.max_visible == 0 {
             return;
         }
         self.selected = (self.selected + 1) % self.items.len();
@@ -108,7 +108,7 @@ impl DropdownState {
 
     /// Select the previous item.
     pub fn select_prev(&mut self) {
-        if self.items.is_empty() {
+        if self.items.is_empty() || self.max_visible == 0 {
             return;
         }
         self.selected = if self.selected == 0 {

src/cortex-tui-components/src/scroll.rs

@@ -119,12 +119,16 @@ impl ScrollState {
     ///
     /// Adjusts offset if necessary to make the item visible.
     pub fn ensure_visible(&mut self, index: usize) {
+        // Guard against zero visible items to prevent underflow
+        if self.visible == 0 {
+            return;
+        }
         if index < self.offset {
             // Item is above visible area - scroll up
             self.offset = index;
         } else if index >= self.offset + self.visible {
             // Item is below visible area - scroll down
-            self.offset = index.saturating_sub(self.visible - 1);
+            self.offset = index.saturating_sub(self.visible.saturating_sub(1));
         }
         self.clamp_offset();
     }

src/cortex-tui/src/widgets/scrollable_dropdown.rs

@@ -254,9 +254,12 @@ impl<'a> ScrollableDropdown<'a> {
 
     /// Returns visible items slice.
     fn visible_items(&self) -> &[DropdownItem] {
-        let start = self.scroll_offset;
+        if self.max_visible == 0 || self.items.is_empty() {
+            return &[];
+        }
+        let start = self.scroll_offset.min(self.items.len());
         let end = (start + self.max_visible).min(self.items.len());
-        &self.items[start..end]
+        self.items.get(start..end).unwrap_or(&[])
     }
 
     /// Renders a single item.
@@ -459,7 +462,7 @@ pub fn calculate_scroll_offset(
     max_visible: usize,
     total_items: usize,
 ) -> usize {
-    if total_items <= max_visible {
+    if max_visible == 0 || total_items <= max_visible {
         return 0;
     }
 
@@ -468,7 +471,7 @@ pub fn calculate_scroll_offset(
         selected
     } else if selected >= current_offset + max_visible {
         // Selected item is below visible area - scroll down
-        selected.saturating_sub(max_visible - 1)
+        selected.saturating_sub(max_visible.saturating_sub(1))
     } else {
         // Selected item is visible - no change needed
         current_offset
@@ -482,7 +485,7 @@ pub fn select_prev(
     max_visible: usize,
     total_items: usize,
 ) -> (usize, usize) {
-    if total_items == 0 {
+    if total_items == 0 || max_visible == 0 {
         return (0, 0);
     }
 
@@ -511,7 +514,7 @@ pub fn select_next(
     max_visible: usize,
     total_items: usize,
 ) -> (usize, usize) {
-    if total_items == 0 {
+    if total_items == 0 || max_visible == 0 {
         return (0, 0);
     }
 
@@ -521,8 +524,8 @@ pub fn select_next(
         // Wrapped to start
         0
     } else if new_selected >= scroll_offset + max_visible {
-        // Need to scroll down
-        new_selected.saturating_sub(max_visible - 1)
+        // Need to scroll down - use saturating_sub to prevent underflow
+        new_selected.saturating_sub(max_visible.saturating_sub(1))
     } else {
         scroll_offset
     };