project_panel: Only show sticky item shadow when list is scrolled (#34050)

Follow up: https://github.com/zed-industries/zed/pull/34042

- Removes `top_slot_items` from `uniform_list` in favor of using
existing `decorations`
- Add condition to only show shadow for sticky item when list is
scrolled and scrollable

Release Notes:

- N/A

crates/gpui/src/elements/uniform_list.rs

@@ -42,7 +42,6 @@ where
         item_count,
         item_to_measure_index: 0,
         render_items: Box::new(render_range),
-        top_slot: None,
         decorations: Vec::new(),
         interactivity: Interactivity {
             element_id: Some(id),
@@ -62,7 +61,6 @@ pub struct UniformList {
     render_items: Box<
         dyn for<'a> Fn(Range<usize>, &'a mut Window, &'a mut App) -> SmallVec<[AnyElement; 64]>,
     >,
-    top_slot: Option<Box<dyn UniformListTopSlot>>,
     decorations: Vec<Box<dyn UniformListDecoration>>,
     interactivity: Interactivity,
     scroll_handle: Option<UniformListScrollHandle>,
@@ -73,7 +71,6 @@ pub struct UniformList {
 /// Frame state used by the [UniformList].
 pub struct UniformListFrameState {
     items: SmallVec<[AnyElement; 32]>,
-    top_slot_items: SmallVec<[AnyElement; 8]>,
     decorations: SmallVec<[AnyElement; 1]>,
 }
 
@@ -145,6 +142,15 @@ impl UniformListScrollHandle {
             .map(|(ix, _)| ix)
             .unwrap_or_else(|| this.base_handle.logical_scroll_top().0)
     }
+
+    /// Checks if the list can be scrolled vertically.
+    pub fn is_scrollable(&self) -> bool {
+        if let Some(size) = self.0.borrow().last_item_size {
+            size.contents.height > size.item.height
+        } else {
+            false
+        }
+    }
 }
 
 impl Styled for UniformList {
@@ -217,7 +223,6 @@ impl Element for UniformList {
             UniformListFrameState {
                 items: SmallVec::new(),
                 decorations: SmallVec::new(),
-                top_slot_items: SmallVec::new(),
             },
         )
     }
@@ -369,17 +374,8 @@ impl Element for UniformList {
                     let last_visible_element_ix = ((-scroll_offset.y + padded_bounds.size.height)
                         / item_height)
                         .ceil() as usize;
-                    let initial_range = first_visible_element_ix
-                        ..cmp::min(last_visible_element_ix, self.item_count);
 
-                    let mut top_slot_elements = if let Some(ref mut top_slot) = self.top_slot {
-                        top_slot.compute(initial_range, window, cx)
-                    } else {
-                        SmallVec::new()
-                    };
-                    let top_slot_offset = top_slot_elements.len();
-
-                    let visible_range = (top_slot_offset + first_visible_element_ix)
+                    let visible_range = first_visible_element_ix
                         ..cmp::min(last_visible_element_ix, self.item_count);
 
                     let items = if y_flipped {
@@ -418,20 +414,6 @@ impl Element for UniformList {
                             frame_state.items.push(item);
                         }
 
-                        if let Some(ref top_slot) = self.top_slot {
-                            top_slot.prepaint(
-                                &mut top_slot_elements,
-                                padded_bounds,
-                                item_height,
-                                scroll_offset,
-                                padding,
-                                can_scroll_horizontally,
-                                window,
-                                cx,
-                            );
-                        }
-                        frame_state.top_slot_items = top_slot_elements;
-
                         let bounds = Bounds::new(
                             padded_bounds.origin
                                 + point(
@@ -448,6 +430,7 @@ impl Element for UniformList {
                             let mut decoration = decoration.as_ref().compute(
                                 visible_range.clone(),
                                 bounds,
+                                scroll_offset,
                                 item_height,
                                 self.item_count,
                                 window,
@@ -493,9 +476,6 @@ impl Element for UniformList {
                 for decoration in &mut request_layout.decorations {
                     decoration.paint(window, cx);
                 }
-                if let Some(ref top_slot) = self.top_slot {
-                    top_slot.paint(&mut request_layout.top_slot_items, window, cx);
-                }
             },
         )
     }
@@ -518,6 +498,7 @@ pub trait UniformListDecoration {
         &self,
         visible_range: Range<usize>,
         bounds: Bounds<Pixels>,
+        scroll_offset: Point<Pixels>,
         item_height: Pixels,
         item_count: usize,
         window: &mut Window,
@@ -592,12 +573,6 @@ impl UniformList {
         self
     }
 
-    /// Sets a top slot for the list.
-    pub fn with_top_slot(mut self, top_slot: impl UniformListTopSlot + 'static) -> Self {
-        self.top_slot = Some(Box::new(top_slot));
-        self
-    }
-
     fn measure_item(
         &self,
         list_width: Option<Pixels>,