fix(cortex-tui): fix scrollbar position to reflect viewport, not selection

The scrollbar in dropdown menus was incorrectly using the selected item
index as its position, which caused the scrollbar thumb to jump around
based on the highlighted item rather than showing the actual viewport
position.

This fix changes the scrollbar state to use:
- scrollable_range = total_items - max_visible (the actual scrollable area)
- position = scroll_offset (the current viewport position)

This makes the scrollbar properly indicate which portion of the list is
currently visible, so when scrolling past items like /usage, the user
can see that there are more items above that are now hidden.

Fixed in:
- AutocompletePopup (slash commands)
- MentionPopup (file mentions)
- ScrollableDropdown (generic component)
- SelectionList (generic selection)

Added max_visible() getter to FileMentionState for proper scrollbar
calculation.

Fixes scrollbar behavior from PR #216.

Author: factorydroid

cortex-tui/src/mentions.rs

@@ -291,6 +291,11 @@ impl FileMentionState {
         self.scroll_offset
     }
 
+    /// Returns the maximum visible items.
+    pub fn max_visible(&self) -> usize {
+        self.max_visible
+    }
+
     /// Returns whether there are more items above.
     pub fn has_more_above(&self) -> bool {
         self.scroll_offset > 0

cortex-tui/src/widgets/autocomplete.rs

@@ -279,11 +279,12 @@ impl Widget for AutocompletePopup<'_> {
         // Render scrollbar if needed
         if needs_scrollbar {
             // Create scrollbar state
-            // content_length = total items
-            // position = selected item for proper thumb position
+            // content_length = total items minus visible items (scrollable range)
+            // position = scroll_offset for proper thumb position reflecting viewport
             let total_items = self.state.items.len();
+            let scrollable_range = total_items.saturating_sub(self.state.max_visible);
             let mut scrollbar_state =
-                ScrollbarState::new(total_items).position(self.state.selected);
+                ScrollbarState::new(scrollable_range).position(self.state.scroll_offset);
 
             // Define scrollbar area (right side of the inner content area)
             let scrollbar_area = Rect {

cortex-tui/src/widgets/mention_popup.rs

@@ -270,8 +270,11 @@ impl Widget for MentionPopup<'_> {
         // Render scrollbar if needed
         if needs_scrollbar {
             // Create scrollbar state
+            // content_length = total items minus visible items (scrollable range)
+            // position = scroll_offset for proper thumb position reflecting viewport
+            let scrollable_range = total_results.saturating_sub(self.state.max_visible());
             let mut scrollbar_state =
-                ScrollbarState::new(total_results).position(self.state.selected());
+                ScrollbarState::new(scrollable_range).position(self.state.scroll_offset());
 
             // Define scrollbar area (right side of inner area)
             let scrollbar_area = Rect {

cortex-tui/src/widgets/scrollable_dropdown.rs

@@ -357,10 +357,12 @@ impl<'a> ScrollableDropdown<'a> {
         }
 
         // Create scrollbar state
-        // content_length = total items
-        // position = selected item (or scroll_offset for better accuracy)
+        // content_length = total items minus visible items (scrollable range)
+        // position = scroll_offset for proper thumb position reflecting viewport
         let total_items = self.items.len();
-        let mut scrollbar_state = ScrollbarState::new(total_items).position(self.selected);
+        let scrollable_range = total_items.saturating_sub(self.max_visible);
+        let mut scrollbar_state =
+            ScrollbarState::new(scrollable_range).position(self.scroll_offset);
 
         // Define scrollbar area (right side of the inner content area)
         let scrollbar_area = Rect {

cortex-tui/src/widgets/selection_list.rs

@@ -615,10 +615,13 @@ impl SelectionList {
     /// Render scrollbar on the right side.
     fn render_scroll_indicators(&self, area: Rect, buf: &mut Buffer) {
         // Create scrollbar state
-        // content_length = total filtered items
-        // position = selected index for proper thumb position
+        // content_length = total filtered items minus visible items (scrollable range)
+        // position = scroll_offset for proper thumb position reflecting viewport
         let total_items = self.filtered_indices.len();
-        let mut scrollbar_state = ScrollbarState::new(total_items).position(self.selected_idx);
+        let visible_height = area.height as usize;
+        let scrollable_range = total_items.saturating_sub(visible_height);
+        let mut scrollbar_state =
+            ScrollbarState::new(scrollable_range).position(self.scroll_offset);
 
         // Define scrollbar area (right side)
         let scrollbar_area = Rect {